According to Mab, "very few Knights are strong enough." Harry's strength made him capable of forming the Banner, I doubt Slate would have been able to.
Yes, but what
kind of strength?
Magical brawn? That's credible, as the foundation of a magical "Banner."
Strength of will? Also credible, as the basis of leadership; and a Nietzschean "will to power" is very winter-flavored.
Both needed? Also credible. And, of course, we know Harry's unusually strong with both.
Something else, something more? Given how little we understand about "Banners" (other than the big green irradiated kind), it's entirely-probable there's some ingredient missing from our recipe (tho not likely Gamma radiation).