An NSL can be used only to compel release of connection or transaction metadata, and cannot be used to compel disclosure of message contents. It's basically a fast-track for getting things like call records, and it most emphatically cannot be used to compel turning over a certificate or allowing a man-in-the-middle.

To my knowledge the exact details of the Lavabit case were never released, but from what has been released it's quite clear that the issue was regarding a warrant and a gag order, because the ensuing litigation wouldn't have been remotely applicable to an NSL (otherwise Lavabit's attorney would have won on a walk).

None of this is to say that I think NSLs should exist. In fact, I think they're a terrible idea. But the vast majority of discussions around them and similar topics is so grossly uninformed that it's impossible to take most people seriously on these subjects.