You seemed to have jumped to a conclusion, used the word diagnose, and you are now trying to argue about the use of that word.
The myth is that people realize they are trans because other people told them they are trans, instead of that person realizing it themself. This lets bigots choose between casting trans people as insidious recruiter, seeking out innocent children for conversion, and casting them as helpless victim, too indoctrinated to know what's best for them. Either way, it allows them options for disregarding the views of trans people that that just aren't available when you accept that being transgender is a genuine and considered view of one's self.
No it isn't, there are plenty of people including me who believe that those things are mostly mental illnesses and\or trivial categories invented for novelty's sake. "Objectionable" implies that it's a mainstream opinion that those things are normal and healthy and the fringe opinion is that they're not, if anything, it's the reverse.