tdracer, maybe I'm misunderstanding something, since it sounds like what you are describing is the Calculated fuel that, in fact, did catch the leak and did send an EICAS message, but was ignored.
I find it interesting to work through the logic. Here's what the report says:
At 0521 hrs, the flight crew received a FUEL DISAGREE message on the EICAS. The flight crew performed the FUEL DISAGREE checklist. The FUEL DISAGREE checklist suggested four scenarios in which a fuel leak should be suspected and when the flight crew should perform the FUEL LEAK checklist. One such scenario is when the TOTALIZER fuel quantity is less than the CALCULATED.
And the footnote:
The CALCULATED fuel quantity is determined by the flight management computer by subtracting the fuel used (calculated basing on fuel flow figures as measured by sensors in the engines) from the total fuel quantity at the start of the flight fuel quantity.
So, as I understand it, the totalizer and the calculated (via FQIS) numbers disagreed, and it kicked a message.
The flight crew disregarded the message, because they assumed that the Calculated quantity was the FMS' version of the planned fuel remaining quantity, based on predicted performance according to what planned engine settings were and the aircraft's route. (Does the FMC use Calculated fuel as the basis for its Arrival Fuel Prediction?)
Since the FMS did not have an option for calculating performance based on one engine running at idle, they ran the numbers with one engine INOP.
So they figured that since
A) earlier, they found themselves with 600 kg more fuel than planned
B) the FMS was calculating performance for a route they were not taking, and
C) the right engine was consuming fuel at idle, rather than being shut down, as the FMS thought
that the FUEL DISAGREE message was spurious. They did some back-of-the-envelope calculations and decided they didn't need to do a fuel leak check, presumably because that would involve having to shut their engine down.
In other words, when they got the message, they made up a definition of "CALCULATED" fuel where the calculations were based on the FMS instead of the EQIS. In their imaginary world, there wasn't a problem.
And that's why I'm amused by the recommendation. Someone no doubt who is not as ignorant as I will gladly explain to me:
1. Outside of ETOPS certification, why on a twin with one good engine and near several suitable airports, would it make more sense to reduce a malfunctioning engine to idle than to shut it down?
2. What the benefit is of a fuel leak test with both engines operating, but asymmetrically? How often does a FUEL DISAGREE EICAS message pop up on an already-malfunctioning engine without there being a fuel leak?
In the report's defense, it does make the comment:
During the initial training to operate this aircraft, the operator provides training to all its pilots to understand the requirements of the FUEL
DISAGREE checklist. However, in this case, the flight crew appeared to have misinterpreted certain requirements of this checklist even though they have undergone the training.