In MsgValueSimulator, fallback can only be called with the systemCall flag set (not the ABI one, which is retrieved through SystemContractHelper.getExtraAbiData)
ISystemContract, lines 15 to 21
However, in EfficientCall, function rawCall does let the door open for doing calls to MsgValueSimulator with the _isSystem flag set to false even if the corresponding ABI parameter is hard-coded to true.
EfficientCall, lines 139 to 146
Consider just using MSG_VALUE_SIMULATOR_IS_SYSTEM_BIT, as the _isSystem parameter is expected to be used for non-zero value calls.
As seen in SystemContractsCaller, lines 103 to 112, the comments are the same, so it seems it was copy-pasted.
