Author here. The type of simulation ruled out is "external-soft-error-prone simulation with (two other properties)". If the simulator uses ECC RAM, then the simulation is not external-soft-error-prone.
You might say: "But surely the simulator does use ECC RAM!" Yes, but that's speculation. It's nice (and surprising) that a concrete experiment can empirically support it.
You might say: "But surely the simulator does use ECC RAM!" Yes, but that's speculation. It's nice (and surprising) that a concrete experiment can empirically support it.