This is wild speculation that appears completely unsupported (they know any of this how exactly?), makes no technical sense (rotating/time keyed passwords? yeah it’s possible, but why?), makes no logical sense (more like a suicide note than insurance policy, it’s screaming kill me please to any foreign power that’d want said cache - especially when he’s in Russia) and is coming from the NSA - who don’t exactly have a lot of credibility lately.
Apparently the new strategy to deal with Snowden is to simply make stuff up to accuse him of because his actual “crimes” aren’t all that worrying to a lot of people.