The 'incel' term is a specific identity though: it refers to both the person's romantic circumstances (involuntarily celibate) AND certain beliefs about the causes of those circumstances (blaming women/feminism/Disney/Tinder/etc...). This term was coined years ago, though, from INSIDE the community. I remember reading articles about people calling themselves 'incels' in the early 2010s (can't remember exactly where, maybe Psychology Today). This was before it became a huge community on Reddit/4chan, before the shootings, and long before it became a term used in major newspapers, magazines, and TV news shows.

Even back then, there was differentiation between people who are just involuntarily celibate and those that identified themselves as part of the 'incel' community. The easiest way to tell is to ask why they think they're involuntarily celibate: the 'incels' typically blamed outside societal forces (see above), while non-'incels' talked about their own issues ("I'm shy", "I have social anxiety", "talking to women scares me", "I'm not good at talking to people", etc...).