It's a probable future.
King Thanos saw him wiping out Old King Thor while they were old and separately.
But the Black Winter showed Thor dying at the hands at non-old Thanos with zombified characters behind him.
Then the events of King Thor showed him battling Gorr and ultimately sacrificing himself.
History of the Marvel Universe showed Franklin Richards being the restarter of the universe with Galactus.
And Immortal Hulk had TOBA merge with Hulk to become the Breaker of Worlds and restart the universe.
So nothing really set in stone. It's a plethora of possible futures.