To be honest, I think this is all down to ignorance rather than any deliberate attempt to offend anyone. As someone who identifies as being a gay male, I have to admit that the whole issue of trans rights is still a little bit baffling to me. I've done a fair bit for promoting gay rights in my time and I've met quite a few trans women over the years (mainly because they were cast out of "normal" society for so long and were "lumped in" with the gays and lesbians). For me it was quite easy for me to accept someone who was born as biologically male but had either transitioned to being a woman or were in the process of doing so; likewise I can understand how someone could transition from female to male.
I have to put up my hands and say that the whole non-binary thing is something that I've only really learned about within the last few years and, if I'm being really honest, it hasn't been that easy to get my head around it. It's very easy to assume "he/him", especially on a forum like this where people are essentially anonymous, but because interest in transport has mainly been considered to be a "male" pastime. I do my best to use they/them when I've no idea what gender someone uses, but old habits die hard. While many younger people may take gender fluidity in their stride it's not something I've been used to, despite the fact that I should be more open-minded than most.
It's easy to be offended and decide not to engage the offender in conversation, but until you do so and explain how things are, how are you supposed to get your message across? Screaming "Bigot!" at people isn't really helpful when they don't really know why you're calling them one.