I have thought from the beginning that a deep stall was very unlikely, but it appears that the pilot flying managed to unknowingly stabilize the airplane in a very similar flight condition. I don't know why he changed from a perfectly good pitch and power setting to get so slow, but at that low a speed, the engines were probably providing a goodly amount of nose-up pitch (along with the stabilizer trim automatically coming in to "help" him keep the nose up) so the normal pitch down occurring at stall might not be evident. Other than airspeed, which he has reason not to trust; angle of attack, which isn't displayed?; and an aural stall warning, which is apparently inhibited, how else does he know that he's stalled? What he knows is he has the throttles pushed up, the airplane pitched to 15 degrees or thereabouts (16.5 degrees) which is a familiar number for powering out of a problem, and his rate of descent is at the bottom of the tape and the altimeter is a blur, both high-order attention getters. Who among us wouldn't be confused as to why the airplane was descending instead of climbing and desperately pulling back on the stick to make it climb like it always has before?