Identifying other people as something they're not.

It is without doubt an odd thing to say. Having spent most of my career trying to pick my words carefully in order to be clear but not offend patients when I discussed the risks posed by their weight I used to love it when someone said something like "I know my weight is a problem." It made my life so much easier if someone made it obvious that they recognised that they were overweight and that it did have an impact on their health so I can't imagine ever disabusing someone of that idea (assuming it was true of course.)
That said, you can get your perception skewed with time. Not long before I retired I remember in our pre theatre briefing one day being asked if I wanted to use a piece of bariatric equipment for a particular patient and I replied "No, we won't need it. She's not that big. BMI is only 35." Then I stopped myself and thought "Did I really just say that? Did I actually use the words "only" and "35" in the same sentence?!" But over the course of my career the average weight of patients has risen a great deal, and we have got much more used to managing it, so whereas anaesthetising someone with a BMI of 35 would have been unusual and would have really stressed me say, 25 years ago, it was pretty much a daily occurence by the time I retired a few years ago. So I expect I probably did get used to it and reset my expectations somewhat, but I still can't imagine telling someone with a BMI of 40 that they were quite slim, and I can't think of any of my colleagues who would either. No idea why your doctor said that OP, but in my experience it isn't the norm.

any chance he was being “funny” (otherwise known as rude), or being sarcastic?
I’m in the camp of thinking some (strong emphasis on some) medics are very peculiar indeed.

It’s seems a strange thing to say, was the doctor more overweight than you? Or were you crying and distressed and he/she wanted to make you feel better?
Or alternatively they are used to seeing vastly and enormously fatter people and have a very different benchmark for thin?

I went through a phase of seeing fat people as very healthy and full of life because I had worked in palliative care for a long time and was used to end of life cachexia. It might be a different viewpoint because of their experiences.

Could it have been a bit of profoundly misplaced banter - like when checkout assistants do the joke flattery of wanting to ID you for alcohol even if you are clearly in your 60s?

OP you might find this episode of Gender a Wider Lens with Wesley Yang interesting. It’s v US focused but definitely thought provoking

It's very bizarre. I almost think the NHS is capable of accidentally indoctrinating this consultant into a kind of reality-blind body positivity. After all, we have the example of a male doctor who claims that he is a biological woman, as the result of reality-blind gender ideology, and who appears blind to how that compromises his performance as a doctor.
I wonder whether the consultant was obese and they were attempting to perform their own self-acceptance? But even taking that into account, the experience sounds so unlikely that I wonder whether there could have been a misunderstanding somewhere along the line?

My own NHS experience of INTENSELY STUPID perversions of respectful acceptance of patients' identities is the time I gave blood and was chatting blood groups with a nurse. He said my blood group was "extremely popular", meaning that lots of people had it. When I queried the odd wording, he explained that they weren't allowed to call it "common" in case people found that insulting!!??!!

Possibly he was trying to say that whatever you went to A&E for was not connected to your weight. So why DID you go?

But you said you were overweight so how is he telling you what you think you are by saying you are not?

I wonder how (if?) they treat psychosis. Very bizarre.

So you think a Dr telling you aren’t what you say you are is evidence of years of training that instructs staff to accept what patients say. Is this going to be like one of those Thoughts for the Day when they say “and that’s a bit like Jesus”. Except this going to be “and that’s what it’s like when you tell someone who ‘knows inside’ they’re a woman that they are a man”.

She said it was 40, which is definitely overweight.
The poster has every right to be concerned, no doctor should deny such an obvious health issue!

If the OP is right in what she says, then surely that is very strange behaviour indeed, I would be concerned.

I'm 5"6 and weigh 16 st 2lbs and fall into the obese category going off BMI. Nothing to add just wanted to share how shit it makes you feel

She’s given her bmi.

I think you’re attempting a slight of hand here but I can’t quite work out what you angle is yet. I imagine it will involve some kind of gotcha at the end.

But you told the doctor that you were overweight, so they would have agree with you if it was accepting what you say you are.