In the real aircraft, moving the power lever back below idle has two seperate affects.
The first is to directly control the propeller pitch, reducing it below the forward fine pitch stop position and into a negative pitch position, passing from forward thrust, through zero thrust, to eventually producing reverse thrust. Again, in this portion of power lever movement, you are directly controlling the propeller blade angle only. The engine remains at idle.
Once the maximum negative pitch has been achieved, any further reduction of the power lever only increases fuel flow, and hence Ng%, further increasing reverse thrust by increasing the engine power output.
The net result of both of these functions, is that the further behind the idle gate one pulls the power lever, the greater the stopping forces generated.
Whilst moving the power lever below idle in MSFS still has a noticeable affect, the MSFS simulation of this is greatly simplified, hence the statement in the manual.