Well shit, if some randos on YouTube say it, it must be true. I withdraw my argument in light of this new evidence.
I will diminish, and go into the West, and remain Locutus.
It's either anonymous randoms on some awkward fanfiction site, or Youtubers visible on YT. The fandom and the medium is changing. A term like Mary Sue isn't exclusive to fanfiction anymore. Sure some audiences use the term in a derogatory manner against female characters as a whole, but others use it in a more proper sense to criticize intentionally OP characters regardless of gender.
Sometimes terminology does indeed evolve over time. Seeing how heavily the term Mary-Sue was recently used across the web in regards to Rey, I think it's safe to say 'Mary Sue' is no longer confined to it's original parameters from within the world of fanfiction.