Stupid? Sort of, more, usually deployed in a dishonest/disingenuous way. The claim that ‘cis’ is a slur. People who are talking about trans people being themselves are promoting an ideological agenda are usually themselves wrapped up in their own ideology.
The point is, the more we have words for something the better we can discuss it, talk through it and come to some sort of understanding of it. As soon as you say transwomen but don’t have a descriptor for those of us who aren’t trans you put them apart from the rest of us. So you need to be able to say women inclusive of trans women and when talking about them have a signifier for both those who transitioned and those who remained the same.
I have yet to see an objection along the lines of cis is a slur be deployed genuinely and it annoys me because the people being “gender critical” are doing so based on an ideology that renders my friends and lovers as less than and based in fear rather than any sort of reasoning.