I don’t think that definition of slur is correct. I’d argue that any word that is being used insultingly to refer to a subset of the population is a slur. Using a shortened version of the scientific term without malice to describe someone couldn’t ever be seen as a slur, even if some people didn’t like being called it. Unless it’s common to use it derogatorily, I don’t think that you can call any descriptive word a slur. Since ‘cis’ is just the Latin opposite of ‘trans’, it’s pretty clear that it isn’t meant to be derogatory.

Even if I personally felt offended when anyone called me “tall”, I don’t think I could categorize that as a slur because I am tall and that’s the word that is commonly used to describe my height without intent to offend.