I've used the peapod alone below zero a few times and been fine. I took the top quilt along just incase. I don't think it's necessary most of the time. Might even be psychological- it's nice to have something to drape over you while you sleep.
I don't sleep on an extreme diagonalwhen using the pod, which allows it to fall against my body. I also pull the extra material from the pod into the hammock ontop of me, if that makes sense.
Who knows, maybe I could have got by with a silk inner sheet and the pod. Might have to test it. The fact that Ed rates the peapod to 50F alone means that he must be an incredibly cold sleeper!