Mr. Moore has successfully sold his agitprop to willing dupes. That makes him a successful filmmaker, they bought what he was selling.
That the Academy has also supported his works with Oscar nominations shows more support. It may be more of an embracing of message rather than technique (or even inspite of it) but some arbitors of public taste have said that his work is "decent" or better.
Was Hitler "talented" because he was succesful? No. Talent and success are unrelated. There are many talented people who's work is not recognized in their lifetime (if ever).