I have my own thoughts on trans stuff but I'm still working through it and honestly, on the one hand I am on the same wavelength as both sides.
I agree with concerns raised by all, but on the other hand, it really is a pointless hill to die on. I'm down with flexible pronouns, no big deal. On the other hand, saying stuff like "trans women are trans women" is transphobic strikes me as deeply unhelpful and perhaps even cultish. But that is behind the boundaries of this discussion.
When it comes to One Piece, Oda has established already that people can be gender fluid in his world. It's rare, but the type of gender fluidity in the story is a kind that lets them perform both roles.
Yamato is both the son and daughter of Kaido.
Bon Clay can be a man or a woman.
Ivankov literally shape shifts.
O-kiku can be kiku the waitress or Kikunojo the swordsman.
This is all in the story and pretending that it is not or that this is something Twitter is reading into is completely wrong.
I think most people would be fine with this and see no reason for the deep upset. Where the real world beefs come is things like toilets, sports, people with vaginas, and so on that alienates people. No one is arguing that Yamato should use a urinal, so that's out of the scope here.