It’s a term that’s taken on some additional baggage/meaning. Originally it simply meant someone who was involuntarily celibate - wants to have sexual relationships, but doesn’t. Now it usually refers to someone adhering to a kind of peculiar set of ideologies around that (see: social value theories taken to some often ridiculous extremes; good ol’ fashioned misogyny/perhaps misanthropy; etc.).

There’s a kneejerk reaction to incels in the latter sense because so much that comes out of that is pretty awful. That and it’s often folks who engage with the latter stuff who are more inclined to identify with the term incel - most others who just fit the former definition just say they’re single.

IMO the latter usage is just more proof that we are failing and continuing to fail men, badly, in terms of community and mental health supports.