Dual Gen failure sounds unlikely, they probably had one U/S before departure:
MEL Says you can go with one inop provided the APU gen operates throughout the flight and the back up AC power system is verified to operate before departure.
Luckily the B777 has so much redundancy
(much more than the A330
for example) that they still had the Back Up Generators (BUGs) x2, the APU, The RAT GEN (much higher capacity than the A330 and no "land Recovery"
required) and the Permanent Magnet Generators (PMGs) on each engine shaft.
That level of redundancy is why a B777 costs more than the competition. Maybe at Boeing pilots had a say in the design rather than just engineers and accountants.