Sink rate therefore would have only been 11 fps . . . does not add up.
So what do you think does "add up?
FWIW a clean 777-200 at light weights descending at say Vref 30 +80 (min clean) possibly needs an ROD of around 1000fpm/16 fps with engines at idle to a maintain min clean IAS. However if you chuck all the rules about buffet margins and min FCOM speeds out of the window due to "Force Majeur"/pilot incapacitation then low RODs become very credible and I think extrapolating "time to splash" from some (alledged ) datalink burst becomes highly speculative.
(777 driver with X thousand hours on type, so no doubt about to be moderated out)