ESO has a quest(Daggerfall Covenant, at Glenumbra Moors) where a mage's spell allows time travel and altering history(if ever so sightly..).

Should this not be possible by a basic spell? Isn't time considered mythic/godly in Elder Scrolls?

Could've gotten the spell from a number of daedric lords, Hermaeous Mora being the most likely candidate. Otherwise yeah that's kinda breaks the lore a bit.

This particular period of time seems to have messed with Time quite a bit, so its possible that its easier to manipulate for the moment.

Which also explains some of the books being back there despite not being authored yet.

Could also be the Thalmor unbinding Akatosh in the future is messing things up here in the past.