Things to note:

Propane Air Lower Flammable limit is 2.1%
Propane Air Upper Flammable Limit is 9.5%
Propane Air Stoichiometric Ratio is 6.0%

The generator running on it's designed fuel (petrol) pulls air through the carburetor which keeps the fuel air mixture correct, one assumes some where around the stoichiometric ratio, regardless of fuel air mixture flow rate, that's what the carburetor is designed to do.

When running on Propane I set the propane flow to a constant rate. However the engine speed regulation mechanism is still able open and close the throttle adjusting the fuel air mixture flow rate, allowing more or less therefore still giving speed control.

The mixture however does not remain constant, but as long as the load remains constant the throttle variations are small and the mixture does remain within the Upper and Lower Flammable Limits, and as long as the fuel air mixture will detonate it is usable.

So the answer is the throttle still controls the QUANTITY of fuel air mixture entering the cylinder chamber. Thus it still controls the speed.

Observation
To be honest I can switch the load off and the engine still runs at constant speed. The load as you will see turns out to be only 9% of the total energy I/P to the system so is fairly insignicant...