henra - stopping from 470nm/h in 4 nm is a fairly modest (by automotive standards) 1/3g. Now, for a plane that wants to fly that's an absurd horizontal deceleration. For a plane in an upset condition that may be entirely within what could be expected. A very sharp pullup might do it, at least to my untutored visualization.
This does hint strongly that the upset happened a very short time before the LKP report. A very sharp pull-up followed by a fast drop may explain nobody managed to get to their life vests and people were still out of their seats perhaps queued at the potties. Once the upset started they'd be tossed around the cabin and not able to get back to their seats.
This does favor delta92's scenario, I suspect.