Hi, David!
My take is that, when he heard the beginnings of the break-up from his cabin, he also heard feet above him running towards the stern. (Where was his cabin, by the way?) This seems to tie in with Carl Janssen, on deck, hearing screams from amidships, followed by a surge of people again running towards the stern. Joughin said he got to the Starboard railing on A Deck and stepped off into the water when "the big list to port occurred." Some witnesses and writers have alluded to that sudden tilt, but there's by no means a consensus amongst historians. In any event, once Joughin had stepped into the water and the ship's lights had gone out, as you said, it's doubtful what he could have seen of the stern going vertical, especially being up that close (fancy that!). From there, without being sucked under, and without much physical effort on his part, he was rescued by Collapsible B. Now, also of interest to me is Paul's note regarding a friend of Joughin's, Captain J.H. Anderson, who maintained this version of events was the one which Joughin told verbatim all his life.
Roy