The fuel imbalance check-sheet also points out to make sure that the fuel imbalance isn't the result of a leak, and they did (somewhat -- it was dark outside) try to check for a leak.
They could also have looked at the rate of fuel consumption across both tanks to determine that the fuel consumption on the right tank was way higher than the engine could consume, but that was not SOP at the time (it is now, and the A330 [and probably other planes] now compute that information and alarm on it).
Its all layers, planes, with minimal exception, really don't crash from a single failure.
In this case the pilots followed standard procedure for a fuel imbalance, which is the alert they got.