I don't think the person in the OP is necessarily assuming diagnoses. I agree with you that arm-chair diagnosing based on little information is generally a bad thing to be doing. However, there is a disproportionate number of girls and women with undiagnosed/late diagnosed ADHD. People (understandably) put a lot of weight on official diagnoses, but also, institutional bias exists (though has been improving on this front, in recent years)

This isn't about trusting or distrusting the assessment of doctors. If the daughter in the tweet had been assessed for ADHD and was deemed ineligible for a diagnosis, then I might be more in agreement with your comment. However, the crucial step before being sent for an assessment for ADHD is often an informal one: teachers, family or friends commenting on a child's behaviours and saying "you should get him/her assessed for ADHD". Institutional bias is one thing, but sociocultural bias is insidious and hard to challenge. That's why in my opinion, the person in the OP is doing a positive thing, because the daughter might not have been assessed for ADHD, and if no-one says "have you considered she might have ADHD?", it'll stay that way for many years.

I apologise for the wall of text. My intention isn't to just talk at you, I am genuinely interested to hear your opinions on what I have said, especially if you find some parts of my comment more disagreeable than others.