I thoroughly agree with this point. I have ADHD, diagnosed and medicated as a 49 year old woman. I had always felt 'different' or 'wrong' but am the queen of masking. I have never been considered badly behaved by anyone.
While discussing the diagnosis with my mother, she reckoned she'd never noticed that I'd had any difficulty with life, and that was quite soul destroying because she's right - I masked incredibly well, I'm a high achiever, I managed to fit in but what kills me is that even my mother never noticed what it cost me! God it was hard. Permanent, never having a moment's break hard, hard, painful work.
Now I'm medicated and life is so much easier, I can absolutely understand why some undiagnosed and unmedicated people with the same condition could or would 'behave badly'. Not everyone can keep it up like I did - I don't know how I did and I don't think I could again, should I have to come off my medication for some reason. I just don't think I could do what I need to to 'fit in' again. I can understand meltdowns, burnout, losing control or whatever you want to call it that results in unacceptable behaviour.
No, not everyone who abuses or is rude or otherwise unpleasant is neurodiverse, but I think there is a strong chance that plenty are and for whatever reason never managed to find the coping skills I did.
The answer isn't to excuse the behaviour. It has to come earlier in the form of a societal change that makes it possible to spot those who are struggling, masking, who need help, who maybe need medicating. I don't have the answers to how to make this happen but I can certainly see why those who have had their eyes opened now wonder about others.
I should add though, the people who have to live with this behaviour shouldn't have to put up with it. Maybe the perpetrator needs help and support, but no-one should have to feel unsafe around them.