They'd probably be blasted by the rest of the community for it, but ultimately, they're part of the group being moved which gives them leeway. Outsiders don't get to do that, anymore than it's okay for straight people to say "fag" or white people to say "nigger".

I seriously don't understand this premise that "Did you just assume my X" refers specifically to trans people the way unequivocal terms for some specific subsect of people refer to them.

The phrase is more like "rule of thumb" - without knowing the factoid that it originated as derogatory to women, anyone who'd heard it would just assume (and indeed, many people do) that it means 'general rule'. Just as 'did you just assume my X' sounds like it refers to someone being overly sensitive in general, and you'd need to be told where it came from (or jump to a conclusion based on your own preexisting prejudices) to conclude otherwise.

I don't agree that "using phrases makes people assholes" for not subscribing to the "what pidgin today actually originated from attempted insults towards some group" newsletter - besides which, that does sound like 'language policing' to me. "Of course you can break the law, you'll just have to deal with the standard social consequence of going to prison" is just what actual policing is after all. ("Some people might think you're an asshole" might have been what you wanted to say there, and is less of a 'consequence' than an obvious statement.)