[quote=@Burning Kitty]
<Snipped quote by Nib>

In the MCU version he became the Sorcerer Supreme awfully quick.
[/quote]

Technically, we don't know that he is yet. But regardless, the director/writers have explained that he was trapped in the time loop with Dormammu for a [I]long[/I] time; long enough to achieve a fair bit of mastery.