You see how a single term shuts down your ability to appreciate what he’s saying?
To some extent we do make our own reality, which I think is well illustrated by your reaction to what is a term of literary criticism.
But it is not a term of literary criticism. It’s a way to basically say that OP women are reverse-sexist or have a misandrist agenda or whatever. Let Rey be Rey. If she’s OP for inexplicable reasons, explore that concept, but don’t attach the fact that she’s female to it. Her being female has nothing at all to do with anything, but is at the very core of where the term “Mary Sue” comes from.
The term “Mary Sue” came from the work of a female author who created a character with that name to criticize that archetype. You’re objecting to how you perceive some people seem to mis/use the term.
The argument about the definition of Mary Sue has already taken place in this thread, and not by me. My understanding is the consenus is it is a derogatory term against women, and that the gender-swapped term Gary Stu is just a furtherance of that same derogate against women, since one doesn’t use the term Gary Stu without the context of a Mary Sue. The term is part and parcel tied to the gender-related implication.