He is not making it a slur, it is a slur.
Literally by the liberal rules they try to impose on the average citizen, the person gets to decide on whether something offends them and then condemn the person who used the word.
I find cis VERY offensive, ergo it is a slur. But beyond that the usage is indeed a slur, every time I hear a letter person use cis it is in the context of a pejorative, belittling the person referenced.
Calling someone a “cis-male” or a “cis-female” is on the same level as calling someone an “ultra-MAGA”. It is meaningless and avoids the reality, either you are, or you are not. Adding prefixes in no way alters the basic fact.
And both a “trans-female” and a “trans-male” are physical impossibilities, on a par with a clean turd. There is no such thing. There are only cross-dressing individuals engaging in a cosplay, or they are meaningless mutilated freaks reduced to the state of a neuter.