From my understanding, and I could be wrong here, Bethesda only considers content "official" or "canonical" if it's part of a published work. Developer posts, whether made here or elsewhere, may not
technically be official but I think they're close enough to be accepted. It's very murky, if a developer clarifies matters is that different than releasing new lore? What if it does or does not contradict any existing lore? What if the content comes from the library of unreleased TES documents?
I think this murkiness is sufficient to keep the content from being
official but with developer comments it's something of a technicality. If Todd Howard came in and clarified something about the disappearance of the dwarves I think people would be generally OK accepting that information as true, and if not wholly
official now likely to become so in a future release.