We knew that since SoD had come out.

And I doubt The Dark One is the only one who can come up with a ritual around the gates. I'm pretty sure any divine (near)epic caster could develop some unique ritual for fiddling with those gates (and I am very sure the other gods/fiends could as well).

Vaarsuvius has been much longer than 8 hours on the island, so whatever has happened with the Big Devil isn't important here. Vaarsuvius squandered many spells in the fight with the Imp (which is when the Dragon went in to attack).