Not saying it's confirmed, but it makes sense. It'll require certain events to happen in the lead up to the finale, but it's been properly foreshadowed (plus why Baby up the Night King's face from season 6 onwards, when D&D possibly learned about it). It also might go down a little different than the theory thinks.
But we know there's at least one more "Holy crap!" Twist on the level of Hold the Door. D&D mentioned GRRM told them three things that really shocked them, and they said one was near the end (which is when this would be).
Itd definitely cause that bittersweet ending. And I'm pretty confident there's some sort of curveball about the Walkers. GRRM wouldn't plan the entire final book to be a good v evil showdown, it'd be boring.
It's also an arc that'd only need 3 - 4 episodes to really tell. It'd definitely fill up some of the time next season so it's not all "Humans vs White Walkers"