Yes, I imagine that Campbell's claims could easily be overbroad. (I haven't read his book.) And convergent evolution happens in technology and in writing, just like in biology! But I think guideline is probably not a strong enough word--my understanding is that Campbell's claim is that there's a combination of Jungian archetype(s) and also that we don't realize how constrained we are to write stories that are similar to other stories we've read.
It's been said that every calculus text ever written has been either a copy of Euler or a copy of a copy of Euler. We find it hard to see new ways of doing it after seeing his way. So, I think that instead of viewing The Hero storyline as a guideline, we're meant to see it as a distillation of what he saw writer after writer after writer doing. It was description, not proscription.
As I write this I wonder if I'm taking 'guideline' as you meant it! I see it as a proscriptive term.