Of course self diagnosis is valid. But you can’t expect everyone to accept it - that’s just life, but people do not have the right to tell you that you are wrong either.
However without a formal diagnosis there are no legal workplace supports, access to certain services (I live in Australia so YMMV) no access to certain medical services and medication and other things that actually require a formal diagnosis.
I am dubious though when people self diagnose but refuse to tell anyone other than the internet about it because they feel they will be discriminated against at work or by their peers…..because for the rest of us who ended up with a formal diagnosis as a child we didn’t get that choice…so in that sense that view is a little bit privileged in that you get a choice many in the rest of the community did not….