I could be wrong, but I think there's a specific violent threat embedded in calling trans people nazis - nazis are still the simplest shorthand terminology for "unambiguously evil oppressors whom it is acceptable to violently oppose." No one wrings their hands and bemoans the Allies' treatment of the nazis in WW2. To this very day it's hard to answer "is it okay to punch a nazi" negatively- most people would probably agree that the nazi had it coming.

That kind of sentiment makes this whole "trans nazis are infiltrating and controlling the cis lesbian and gay rights movement" accusation a little scarier, in my opinion, due to context - portraying your ideological opponents as nazis as prelude to violence, and trans people are already the target of plenty of violent transphobic attacks.

(I still don't think this arrest is warranted, absent some sort of other evidence that he intends to do more than call people names online... but there are very specific connotations to calling trans people nazis, that I feel like it's fair to call out aside from mere 'political beliefs')