I think their point is that it's a Z and not the Russian tricolor (not sure how you'd make the latter a rainbow anyway), i.e. an endorsement of the campaign specifically and not the state generally.
I still think OOP is goofy because neither the Russians nor the Ukronazis are fighting to protect or advance trans rights, but I don't think it's exactly as bad as using the flag of a queer-phobic state, be it Russia, Ukraine, or America.
