Comment by Kadin

I've heard plenty of good chemists refer to monatomic gases (and sometimes ions) as molecules, or "molecular", colloquially, and particularly when talking generically in terms where they could be replaced by another gas. That helium is monatomic is both well understood and not especially relevant in many contexts.

He2 is rare enough that the few times I've heard it spoken about, people have usually said "helium dimer" to make it clear that's what they mean. Since that's the rare case, it's the logical one to be explicit about.