"But loss of instrumentation in a storm might." True, the Airbus flight out of South America a number of years back bears that out.
What I'm saying is that most aircraft have some kind of glide slope (except helos) and even with engine loss, the APU should provide power and backup emergency hydraulics to control the aircraft.
https://en.wikipedia.org/wiki/Gimli_Glider
Nose diving in just brings us to other possible scenarios.
The Gimli Glider crew were blessed with clear air, good visibility, just no fuel. The absence of severe air movements found in thunderstorms doubtless helped, although they could have used quite a few more liters of fuel...
IIRC, the time for power up from dead to the APU kicking in is roughly two minutes.
A plane could get in a lot of trouble in those two minutes, and they didn't start with a lot of air between them and the ground.
Think sudden up and down drafts, turbulence, and no visual reference, no instruments....120 seconds is a long time.
We will know more when the flight recorder data comes out.
From your link:
The 767 was one of the first airliners to include an electronic flight instrument system, which operated on the electricity generated by the aircraft's jet engines. With both engines stopped, the system went dead, leaving only a few basic battery-powered emergency flight instruments. While these provided sufficient information with which to land the aircraft, a vertical speed indicator—that would indicate the rate at which the aircraft was descending, information which could be used to predict how long it could glide unpowered—was not among them.
On airliners the size of the 767, the engines also supply power for the hydraulic systems, without which the aircraft cannot be controlled. Such aircraft are therefore required to be equipped with a means to compensate for this kind of power failure. With the 767, that compensation is usually achieved through the automated deployment of a ram air turbine, a backup generator that generates power from air movement, like a wind turbine.[9] As the Gimli pilots reduced speed for landing, the resultant reduced airflow meant a decrease in the hydraulic power, power that was critically needed for control during landing.
At issue is whether the engines were providing power during the sharp descent. If so, then the hydraulics and instrumentation should have also been functional, and that would lead us to other possibilities when it comes to the sharp descent.