I would wonder whether one should rely on any sim to provide usefully valid information for such a scenario
Why not?
The behaviour of a big jet in case of a total loss of thrust should be easily predictable. It's not like it's flying upside down or in some aerobatic maneuvre at the edge of the aerodynamic envelope of the plane. The simulator is able to simulate the plane's behaviour in case of any change of thrust, so why should it not be able to simulate a change of thrust to zero? It is able to do that in a one engine out scenario, so why not with both engines out?
The only effect which probably isn't simulated that accurately is that of the windmilling engines, because most likely it hasn't been tried in the real plane, but everything else should be replicated exactly, I believe?