To be fair depression has a big range.
An analogy off the top of my head (not perfect, obviously, and there are lots of differences) is that it could be compared to something like asthma vs lung cancer, in that asthma can range all the way from
- mild, occasional, maybe partly outgrown, easily treated, and with without significant impact on your ability to live your life as you want, to
- severe, hard to treat, extremely limiting and disabling, and potentially life-threatening.
Whereas you pretty much can't have lung cancer without the disease and/or the treatment having a significant impact on your life, and, even with the more readily-treated lung cancers, there's still a high chance you won't survive it.
Someone who dies from asthma is just as dead as someone who dies from lung cancer, but it probably wouldn't be useful to group someone with mild asthma and someone with stage 1 lung cancer together.
Obviously the analogy I made doesn't match in every way — the similarity I meant to highlight is just that different mental illnesses can have a different range of likely severity of impact, as well as different types of symptoms.
But this comparison doesn't even take into account the broader use of terms like depression/depressed and anxiety/anxious in everyday speech.
The word "asthma" isn't often used in everyday speech to talk about a self-diagnosed wheeze, or to refer to normal experiences like feeling out of breath when you run or coughing when you inhale smoke.
But people do sometimes self-diagnose with depression or anxiety for whatever reason, though may not always technically meet criteria for the medical conditions of generalised anxiety disorder or major depressive disorder or whatever (or may have a different disorder). And I think even more commonly, without even intending to self-diagnose, people use terms like "depressed" and "anxious" in their ordinary, everyday meanings, to refer to normal (if unpleasant) emotional experiences.
So when talking about depression, there's a massive range of things that can mean, from severe major depressive disorder which can include psychosis or catatonia, through moderate and mild clinical depression (and other related disorders like dysthymia, adjustment disorder, complicated grief etc.), through people who are struggling with their mood but may not technically meet diagnostic criteria, to people who just happen to be using some of the same words but who mean it in an everyday sense of feeling glum, low, fed up, flat, blue, down, sad, dejected, demotivated, or whatever.
Depressive disorders (and anxiety disorders) are some of those mental illnesses that have a big range of expression (and like asthma can range from totally manageable to deadly) — in some people they can be mild or temporary or a one-off, can be managed well with easily-tolerable treatments, and/or can be accommodated without severely affecting a person's ability to function, while other people with the same diagnosis can experience severe, lifelong, intractable and/or deadly illness.
Then there are also mental illnesses which, when a person is diagnosed with them, mean that that person is very likely to have significant symptoms and may struggle with even basic daily living tasks, sometimes even with the best treatment available.
For people with these diagnoses, aside from the suffering that results directly from the mental illness, they'll often have to endure highly unpleasant treatments such as medications that cause organ damage, permanent movement disorders or diabetes, involuntary hospitalisations with forced medication, intrusive and painful psychotherapy, enhanced state involvement in their life, and so on. There's a high chance their illness will persist or recur throughout their lives, and they have a strongly elevated risk of experiencing other illness or disability, discrimination, loneliness, poverty, and early death. Not everybody with these diagnoses experiences these, but some mental illnesses are associated with much more difficulty on average than other mental illnesses.
Severe mental illnesses are often considered to include eating disorders, personality disorders, schizophrenia, bipolar disorder and other psychosis. (With the last three diagnoses, your GP will put you on a SMI register, so you get targeted for extra health checks. It's not that other mental illnesses aren't or can't be severe, it's just that the psychotic disorders are the ones they thought worth targeting for extra health checks, as many of us die young from physical conditions.) Other mental illnesses can be severe in some people (so severe depression is a severe mental illness) but aren't considered to be intrinsically severe, in the way that any diagnosis of bipolar disorder will automatically get you added to the SMI register.
When GreenTea said it is actually not useful to have "everyday depression" and "psychosis" lumped into the same category I think she was referring to this issue of the range of meaning, specifically with regard to "mental health problems".
I don't know whether she used the term "everyday depression" to mean mild-to-moderate clinical depression, or to those experiences people refer to with the words "depressed" or "depression" which are either normal emotional reactions or which don't meet the threshold to be diagnosed with a mental illness.
Either way, I don't read her comment as dismissive of depression — I read it as an expression of frustration at the inability to easily define the group you're discussing, when the term commonly in use can encompass serious mental illness (including severe depression), other less severe or impactful mental illnesses (and mild depression is less severe and impactful than severe depression, or schizoaffective disorder), people with difficulties not diagnosed as mental illness, people struggling for any number of external reasons, and people experiencing normal healthy emotional responses to normal life events and stressors.
GreenTea didn't say that depression "isn’t a significant mental illness", rather I think she was referring to something that's become a real problem in the popular "mental health" discourse.
People with mental illness have all but disappeared from the conversation, especially those with severe mental illness like schizophrenia, anorexia nervosa, and yes, severe depression.
There's awareness-raising and discussion everywhere but it's all about "chat to your mates, they might be feeling down" and "if your friend always shows up late just ditch them altogether, you've got to have boundaries to protect your mental health" and "I need my emotional support dog because I've got mental health" and "mental health tips to help you through exam time", and maybe the odd sanitised personal story from someone with one of the more socially palatable, relatable diagnosed mental illnesses.
There's no real way to discreetly indicate the fact that I have a mental illness or that I am currently experiencing a relapse without being explicit about my severe mental illness. It used to be that if I said I had mental health problems, people knew that was referring to a psychiatric condition without me having to disclose specific stigmatised conditions. If I mention "mental health" now, there's nothing to differentiate someone with bipolar disorder from people who like to take a day off every so often, or someone who's feeling sad and lonely because their girlfriend has gone away for two weeks.
But all this awareness-raising has done very little to mitigate the discrimination I'll experience if I just come out and say I have bipolar disorder. The push for conversation about mental health pretends to be at least partly for me and people like me, but it's almost entirely for people with mild to moderate common mental health problems, as well as people who are mentally healthy and could benefit from some wellbeing tips.