I made this experience so this is representative

That is not what they claimed. They claimed to represent "many", not everyone. Only you made that claim.

You can tell them that their argument is "an anecdotal fallacy" for which "no sources have been cited" and ask them to provide further evidence. That is not what you did. (You responded with your own sourceless anecdote, followed by the aforementioned.)

attack the idea that invalidating self diagnosis is gatekeeping (which will probably fail) and not me personally

You seem to have misunderstood my intent.

I don't really have a strong opinion about people who self diagnose. It does not matter to me whether or not they're freely included. As such, I do not intend to "attack the idea that invalidating self diagnosis is gatekeeping". As I said, you're free to have that opinion, I won't challenge it.

The thing I do have an issue with is how you presented your opinion as without alternative, claiming to represent the autism community as a whole and disregarding others' points of view.

You can try to invalidate my point, insulting me by saying that criticising your way of presenting and discussing views "shows lack of character".

But that won't stop me from telling you to please stop unreasonably misrepresenting and disregarding others in the future.