That wing should glide fairly well - I just read somewhere that the ATR has a L/D of around 17:1.
Not sure of how valid or relevant this may be, but some (very) approximate calculations from those traces suggest in the minute between 03:53:30 and 03:54:30 they descended approximately 600', and taking a stab at an average speed of say 90KIAS, this represents 450' / nm, equating to an L/D of around 13.3:1 (assuming no thrust).
Do the ATR's props decouple under high neg tq?
Maybe someone far cleverer than I could say if anything can be inferred from this?