Why do they do this?

Brother-In-Law has ASD and probably ADHD too based on behavior. 

We have often clashed over his argumentative nature, particularly about politics on social media where he thinks that the world can be saved by aggressively berating people with his views. 

The real souring point was when I discovered that he had been texting my wife outside of social media about what an idiot I was for not conceding his correctness in one of said arguments. I told my wife that was bizarre and a profound overstepping of boundaries on so many levels (she was sick of it too) and she told him to stop. Thankfully he has respected that boundary. 

We have twins who are not even 2 yet, and last night, my wife was describing that our daughter likes to engage in play often by stacking like things. She is, as of now, perfectly on track as far as developmental milestones. His response to the stacking play was "That sounds like an ASD behavior."

He has an MS in counseling - no job in his field in almost 2 years since graduation because he wants the perfect job without having to cut the cord and move away from his support system - with some work with an autism center. Nowhere near the expertise to actually diagnose others. 

This isn't the first time he has "diagnosed" people (i.e. projected his diagnoses on to others) and he's not the only ASD/ADHD person that I have encountered who loves to do it. 

Is this a belonging thing? Like "I can normalize and advocate for my condition by finding as many people like me as possible?" I find it obnoxious and bizarre.