The term is fine. There's nothing improper about using "-ism" to denote a stance that promotes or is favorable towards or simply contains a lot of something.

Absenteeism is a thing, despite people either being absent or not. Catholicism is a thing, despite people either being Catholic or not (at least according to doctrine).

There is a posture with regard to how to view transgender individuals as associated things like gender, and it is reinforced along political lines.

That doesn't mean it's a useful concept. It doesn't mean that the concept isn't used to mislead. And, of course, as with any term, people will sometimes use the term incorrectly.

But as far as terms go, it's not very faux. It's perfectly comprehensible and adheres to standard usage of the "-ism" suffix.

However, the implication, because of how and by whom the term is used, is that if you're saying "transgenderism" you probably think that there is nothing but ideology which is demonstrably wrong. It's not because "transgenderism" is a "faux term" in any important sense: linguistically, it's quite reasonable. The problem is the associated beliefs of most everyone who makes a show of using the term.