maybe he doesn't know Garab Dorje was a "Buddha".
I had an inkling, but not 100%.
Did Garab Dorje learn the Dzogchen teachings from a Buddha in the Sambhogakaya dimension?
Yes, from Vajrasattva. Some accounts have Garab Dorje simply as a nirmanakaya emanation of Vajrasattva, thus not in need of learning anything as "Garab Dorje". Either way, yes, Dzogchen comes from Buddhas, whether in Buddhism or Bon.
But Dzogchen is definitely a Mahayana system, which only means that it is inseparable from the bodhicitta motivation to liberate all beings. Mahayana includes exoteric (sutra) and esoteric (the tantric paths of transformation and self-liberation) aspects. I don't think Berzin comments at all on whether Dzogchen is found on other world systems, though. I've never seen him be anything but entirely respectful of other Buddhist paths, and since important Dzogchen tantras like the Dra Talgyur definitely state that Dzogchen is found elsewhere in the cosmos, I doubt Berzin would dispute that. Especially since Mahayana sutras commonly speaks about Buddhadharma being practiced in countless other world systems as well. Further, Berzin may well practice Dzogchen himself, though I have no idea. The one thing I can say about his discussion of Dzogchen is that it definitely sounds like Dzogchen explained in a way that a Gelug practitioner could relate to. Of course, for a Gelug audience, maybe that's a skillful means.