In the generally accepted definitions of denomination I don't see anyplace that says "only applies to christians".

There are two major denominations of Jains, at least five of Bhuddists, I could go on but I think you see the point.

In common usage, people say "non-denominational christian" (the second word qualifies the first) or "non-denominational bhuddist" and that's fine, but if you are "non-denominational" without additional qualifications you can't also be "christians only".

OK, I think I pegged the theology geek meter on that one. Sorry! Just ignore me.