I mean, to be fair, Draenei worship the Light and revere the Naaru. Humans got taught about the Light by Naaru visiting them in dreams. This doesn’t mean humans and draenei are descendants of the same race.
That said, I think there’s enough evidence that supports the troll-elven relation. However, from an in character perspective there really isn’t that much evidence. Trolls claiming it could be dismissed as them wanting to hold something over elves, and Brann really only theorises it. And as far as I’m aware “Chronicles” isn’t a collection of books that exists in the world, so we haven’t had Titans or titan creations claim this, either.