Firstly the 777 is Fly by Wire
If all Generators and PMG's ( both transfer busses are un-powered ) are lost the Flight Controls will run on DC power from the main battery. Even then they have their own batteries for 1 min while power is transferring around from Battery to RAT.
The APU ( if serviceable, and it is 99% of the time ) Will Automatically start in about 1 min
So.........
Seems to me, looking at the pictures: Both Engines flamed out at around 500'
There are no spoilers deployed, indicating loss of Hydraulic power.
The APU inlet door is open indicating that it was in the process of starting.
Cannot see if the RAT is deployed, but it will be I'm sure.
With the Engines failed and quiet the loud buzzing noise heard by the witness WILL BE THE RAT, it makes quite a noise.
The Crew did an amazing job of using what little excess energy they had available above Vs to reach the grass. Damn fine effort.
Now as to why they lost Power???????????
That's what we need to wait for.