Self-diagnosis is really common in the ND community, but as an AuDHD adult myself, it irks me.
I don't want to be a gatekeeper and I'm only too aware of the difficulties with diagnosis. I wouldn't normally say anything in real life, but I'm aware that I feel a frisson of irritation when met with a definite declaration of neurodivergence that isn't backed by a proper diagnosis.
I have no issue at all with anyone who says "I think/believe that I/my child is probably autistic/ADHD". In fact, I have a lot of friends with ND-diagnosed children who then realise that they are probably also ND themselves.
I think it's multiple factors.
Firstly, my autistic brain likes things to be correct. And a declaration that you ARE DEFINITELY autistic/ADHD etc when you haven't been diagnosed is nothing more than an informed guess. It feels "wrong" to me to tell people that you have a condition that not only overlaps and mimics other conditions, but is also quite challenging to diagnose. You wouldn't do that for any other medical condition so why is it acceptable for autism/ADHD?
I think the increased awareness of ND conditions means it's almost become trendy with loads of TikTokers declaring themselves as ND for a whole host of ignorant and uninformed reasons. And that type of ill-informed behaviour reflects badly on genuine ND folk because we're then all assumed to be charlatans - and we get enough of that already.
Thirdly, if it's about a child, there is in the instinctive reaction that you need to put in the bloody work to get the child what they need! Support should be based on need, but we all know that having a proper diagnosis makes it so much easier to qualify for support, and to get people onside who really believe that your child is ND. It isn't easy pulling everything together to get a case seen so parents side-stepping that feels lazy and not in the best interests of the child. And yes, that's frustrating, unfair, and really not OK.
Lastly, some self-diagnosed people bandy their declarations around like flag, waving it in everyone's face and demanding that excessive exceptions are made. It's a curious type of behaviour that you see far less frequently in the parents of children who have been diagnosed. I don't know if it's a lack of understanding or they're just trying to convince themselves, but telling the world and their dog about your child's neurodivergence is unnecessary and really encroaches on your child's right to privacy. And yes, unfortunately there is a small minority of parents who use their child's neurodivergence as a way of getting attention for themselves "see how difficult our lives are". I have seen Facebook posts from parents who have shared videos of their child in a meltdown or photos of bruises on their arms to prove how hard life is for them. Just fuck off! A neurodivergent child isn't an opportunity to get attention.
All of that aside, I believe that behaviour is communication and that every child needs to be listened to, and appropriate support provided, regardless of diagnosis. Your nephew sounds as if he's having a really hard time so he needs to be on the diagnostic pathway so medication can be considered. I'm not sure I've nailed explaining the issues with a lack of diagnosis - and I'm not 100% sure I really know entirely what it is myself. But at its core, it's frustrating because it isn't putting the child first and has the potential to damage the rest of the ND community.