Linguistically they're prefixes I mean. You're right, when used on its own it is an abbreviation but within transgender or cisgender they're prefixes. It's a relatively new thing to use "trans" or "cis" as an abbreviation instead of a prefix, so it feels natural to turn it back into a prefix by attaching it to the next word, and "transwoman" and "ciswoman" still kind of work as long as you do both because cis- and trans- are modifying the womanness. I agree that even that is uncomfortable and othering though, it's definitely better to use trans as an adjective on its own and not divide women/men into separate subcategories based on transness. I just am more understanding of that particular faux pas because I get how people come by it.

"Transpeople" on the other hand doesn't work the same unless you're referring to those who are trans-person and don't identify as people, which I imagine is not who these people are referring to on purpose and rather they are dehumanizing us as a whole. Both are bad, but I don't think they're equivalent.