There seems to be a small but vocal trans community that wants to turn "cis" into an insult,

Really? TBH I've up to this point only heard of this concept of cis as an insult from cis 'allies', so I was assuming it was just coming from discomfort of being given a label they didn't really understand. But I guess it does make sense that there would be anti-cis radical extremists out there somewhere. I tend to avoid the more hate-filled pockets of the community, so I often have no idea wtf they're saying.

But yeah, in general it's just a descriptor.