The difference is that The Elder Scrolls is a Bethesda product, so any retcons are legitimate if done by Bethesda. Zenimax Online are not Bethesda so their retcons are not legitimate.
It's little more than fan-fiction.
But Bethesda also worked ESO, and I find it highly unlikely that they didn't at least have a hand in creating the storyline.
I'm not sure why the discussion over ESO lore is on going. MK himself has said he considers Elder Scrolls lore to be "open source", in numerous interviews the studio manager for bethesda softworks has said how much he loves the fan fiction and "expanded universe" that fans create, and we ourselves haven't said we're not going to use ESO lore, simply that it takes last place when considering what to do with certain aspects of the mod.
I'd imagine because some people still view ESO as legitimate lore-wise because, well, it's a new game made by Bethesda and Zenimax and should be treated just like any other new game as far as lore goes. Then there's the "it's fan-fiction" camp, whose arguments are based on Zenimax playing a part in making it.
Personally, I'd love your team to adopt ESO lore, for the Longhouse Emperors and Three-Banners War, and how fun they'd be to play.
If you're asking why the discussion is happening HERE, your guess is as good as mine.