Why not? Is Trumpeteer or Biden-ite an offensive term? Why would someone be offended to be associated with something they support?
I support Biden as much as I can considering my position, so it's fine to call me that. Something I notice with Trump supporters is they seem to deny that support when confronted with it and instead hide behind "supporting America" or "It's not about Trump, it's about Patriotism". At the same time, most they say (or in this case, post) screams "I support Trump".
So yeah, if I support Biden I am a Biden-ite and if you support Trump you're a Trumpeteer or any other of those kinds of words. Tell me what's offensive about that?
And if you think someone calling a whole group of people a gay slur for simply having a difference of opinion than them is "pretty civil", I would say that doesn't exactly put you in a great light. Also, I think debating in bad faith by moving the goal posts and perpetuating many of the myths the last presidency thrived on is not civil. The words have been polite but some of the debate, at times, has been anything but.