as i have personally done many many hundreds... if not many thousands of stalls in an airplane i would have to say it is darned difficult for me to imagine a submarine going to a temporary state of full negative buoyancy and sinking like a stone for several seconds simply because of the dive planes going full up.
i have to roll my eyes at the idea.
if you go full up on the dive planes the submarine is not going to arbitrarily sink for a few moments based on the planes position alone.
let me put it to you this way... if you can "stall" a u-boat in the same sense as you can "stall an airplane... shouldnt you also be able to
loop a u-boat with dive planes alone?
the hard thing to wrap the mind around here... is that subarines and airplanes operate on a completely different set of principles