How much does he know about the Severing? Absolutely nothing. Still, Strange isn't going to tell Lambert that much. The fact that he doesn't know anything pisses him off something tremendously, especially since he's trying to seem like Mr. Hotshot Mage who's on top of everything and certainly isn't floundering as much as everybody else.

So, he's going to take a guess. "I assume it's powered by the strength of the mages and the Circle's collective magical will."

Swing and a miss.