I'm guessing you don't know a huge amount about how GPs diagnose mental health problems (which is understandable if you don't work in the field or know where to start).
GPs tend to use the PHQ9 questionnaire to assess depression and GAD7 to assess Generalised Anxiety Disorder. It is extraordinarily easy as a patient to tick 'nearly every day' on every question and come out with a 'score' of severe depression/severe anxiety. GPs have ten minute appointments, including patients getting to the room, taking a history, exploring symptoms (which includes the above questionnaires), enquiring about the patient's goal for the appointment, discussing treatment options, discussing risks and benefits, and then prescribing.
GPs also are sadly known to overdiagnose. For example the time I went to the GP to be signed off for a few weeks due to severe chronic pain (work advised me to do so as my sick record was atrocious due to being off for a day, struggling in for half a day, off for two, back for two, etc.). They asked how things were going and I mentioned that I'd lost a parent two months ago but I was coping okay with it, it was the physical pain I couldn't handle. They gave me a PHQ9 to fill, which I did. It scored me as having symptoms of depression. I explained the answers I gave were due to the impact of the pain (questions include asking if your sleep is impacted which it was, whether you feel like a failure/you've let people down, which I did about work, whether you have had little interest or pleasure in doing things, which I did because I was constantly in pain, and boom that was enough to take me over the threshold).
I did say very clearly I'm not depressed, I'm bereaved, but I'm not here for my mental health, I don't believe I have depression, but it was easier for the GP to send me away diagnosed with depression than to investigate the pain (which later transpired to be stage IV endometriosis), and to prescribe antidepressants, which I never took. He was absolutely incapable of parsing out depression or bereavement and listening to a patient instead of going solely by a standardised questionnaire which is easy to game, extremely sensitive, and designed to score as many people as possible as having depression (was designed by a drug company who could then prescribe for it).
Then there's the fact that if a patient attends the GP, scores on the questionnaires and says they have anxiety and depression, the GP can't disprove it. So they have to diagnose.
I think the proposed changes are a really positive thing. Work is good for mental health in many ways, and it's shocking that as a country we have such a huge percentage of the population claiming they're unable to work, often for things that can't be proven or disproven (like mental health problems).
My spouse is a GP and welcomes these changes, as do his colleagues. It breaks their heart seeing 18yr olds come in stating they can't possibly work because of their MH and expecting to be signed off as unfit for work, not learning those crucial life skills early on, being thrown on the scrap heap. They're also aware that many patients are taking the mick and could do some form of work. But with such truncated appointments and the threat of complaints towards the practice or medical register, and using tools they're expected to utilise which are easily gamed, they haven't really got a chance.