You are mixing up several things at once, which is confusing your point that you offered to others, and causing them to react overall negatively, even though you have some correct points (as well as some incorrect ones).
For one thing, we do not get to decide what we are, and while we do somewhat get to decide what words we use to refer to ourselves, there is a line between what we personally want and what society will allow. If someone refers to me as a "human, member of the species Homo sapiens", then I have no proper basis to claim that they are incorrect. Note that I can say that they are correct, but that does not make it so.
For another, whether something is "friendly" or not is not the sole basis for deciding what is vs. is not a "slur" - if someone sent me a message saying "you are a human, member of the species Homo sapiens"... every second of every day, in perpetuity, then that (DDOS attack) is not friendly. Though it is also not a "slur".
"Cissy" is most definitely a slur, no matter how you look at it - the altered spelling, the similarity to "sissy", etc. "It’s considered a slur when..." - no, it just is a slur, period. The wiktionary definition of slur includes the phrase "socially unacceptable", not "personally unacceptable" but socially so. Note that while it does include the phrase "extremely offensive", that is also followed by the word "and", i.e. to be considered a slur something must be both, not one or the other.
Similarly, the other word "cis" - like Homo sapiens - is not a slur, b/c it is not "socially unacceptable" (even if someone finds it personally thus). Perhaps you meant "It's considered unfriendly when..."? But that's not the same thing as it being an actual "slur".
Though you could legit have meant "socially unacceptable", in which case you would be buying into Elon Musk's radical alterations of existing society, as he works to mold it into what he thinks rather it should become in the future. If true though, note that calling someone "cis" - very much unlike calling a straight person gay -has not historically been considered "socially unacceptable". This addition of the word "cis" to become a "slur" is picking and choosing who gets to define what "society" is. Elon gets to pick, and now anyone who uses it - and whatever other words he decides to add also, perhaps "Homo sapiens" will be added tomorrow? - will have to jump through additional hoops if they want to use it, on his platform.
Which btw is an obvious attention-grabbing tactic, just like the article, except he did it first, knowing that the latter would follow. Anyway, Elon did what he did, and the article did what it did, but you and I get to decide what we will do. So I hope these words help as you think about the subject.
Basically it may boil down to: does Might make Right? If not, then the work is upon us to determine what actually does. Though this is far too simplistic: b/c on "his" platform, he kinda does have the right to do as he pleases, subject to federal and international laws, though we also have the right to leave or ignore or speak poorly about his platform too - he has his rights, and we have ours. So perhaps a better question is "should Might make Right"? And again, if not then what would - e.g. should someone be allowed to call me a Homo sapiens, even if I were to be offended by such a term? Or a more apt analogy seems to be: if I were to have transitioned genders, then am I "trans" in that case? What about "removed"?