If scavenge failed then the fuel sensors did too
The subject of the observation was the timing rather than the failure of fuel scavenge. Ice could possibly delay the onset of scavenge until some time during the descent, then melt away and be scavenged as water.
and:
Quote from Boeing relating to 777-300ER (not exactly the same scavenge arrangement, but might or might not have similar lack of instrumentation - any lurking 777 pilots able to comment or confirm?)
There is no indication to the flight crew that the scavenge system has failed nor that the fuel (remaining in the CWT) is unusable.
(My bold formatting, and my italic clarification)