Well, they called it a slur. That's good enough a reason.
That's why I don't like the idea of censoring slurs. Anything can be one.
If some chap at X, determining which word is considered a slur, says, "I watched a YouTube video with telling someone else not to call them 'cisgender'.", that's probably good enough to add it to the list, while most of them not actually matching the dictionary definition for "slur".
The point comes as to where to draw the line and the company gets to choose.
Welcome to modern society. Everybody loves to pretend.
The people pretending to be offended by some random mistaken word uttered by another.
Those pretending to care about something that they are using "politically correct" words for.
Microsoft pretending to care about OSS, in the hopes of getting some highly performant devs.
...
Yes, it's not a slur. But someone told another person to not call them a "cis woman" on camera and now it is whatever, you call it.