I actually see a weird parallel with this thread (and similar ones) and the "MNers are too quick to cry LTB!!" threads.
The arguments seem to be the same - and honestly I think the conclusion (from my side at least) is the same too.
I don't think anyone ever has decided that their child has ADHD/autism/etc or sought an assessment for such things based on comments on a MN thread alone. If comments on MN are one part of an overall picture that caused them to go and seek out an assessment, then that is what they were. And at least IME, sometimes MN is the only place where people will outright come out and say "Have you ever considered...?"
And so no, I don't find it particularly surprising when people suggest ADHD/autism and the poster says "Well yes I have wondered about this but <insert common misconception here> so probably not".
Cards on the table here - I did LTB, 15 years ago in fact, which MN was supportive of and which was absolutely the right decision. I have one child diagnosed with ADHD, that MN never really picked up on, although he is a teenager now - or perhaps I wasn't in that headspace, because I don't remember reading comments like that, despite posting about him a LOT ~10-15 years ago, because I was really struggling. And one child in assessment for both ADHD and autism (MNers picked up on this, though I haven't posted much about him). I am also diagnosed ADHD myself, thanks in part to comments on MN although perhaps thankfully, the one that made me seriously consider it was not on my own thread so I didn't need to endure the back and forth bickering about whether or not it was a reasonable suggestion or whether I was just lazy 
And I have also had occasional comments suggesting that my DH may be abusive/manipulative (which he is 100% not
) and that I may be autistic which I am pretty confident I am not. I also remember it being suggested at least once, maybe twice that DS1 (despite the ADHD never really being picked up on!) was PDA, which I was sceptical about at the time and now he is older I'm certain he is not and has never been PDA.
And perhaps controversial - the ex who I left 15 years ago was roundly "diagnosed" by MN as having NPD. I now find this quite unlikely, but it was helpful to me in terms of helping me emotionally detach from the relationship, which was extremely harmful regardless of whether or not my ex has any personality disorders. Since I never told him of my suspicion or anybody else, it caused no harm in the real world.
Point being - I think it's really OK to have these things suggested, whether they turn out to be relevant, or not. I also don't think "Have you ever thought about X" or "This sounds a bit like it could possibly be Y" is diagnosing someone over the internet. It's not like posters are saying "Oh yes OP definitely ADHD, you should start him on caffeine immediately!" I should think anyone suggesting that would be rounded upon and quite rightly.
There is this idea that autism/ADHD/etc are really rare so suggesting it to people online is like hearing hooves and thinking zebra rather than horse. But actually they are very common - about 1-2.5% is thought to be the prevalence of autism and around 3-5% is thought to be the prevalence of ADHD (with some overlap between those). So it is worth thinking about, the same that for example difficulty in learning to read and write is worth considering possible dyslexia.