That may have been the case with mechanical fuel injection, but it ain't so with electronic controls. I've run prototype fuel systems on diesel engines in dyno cells running on a 90/10 mix of CNG and diesel, and there's no power loss relative to straight diesel. You need to keep a little shot of diesel to light the mix, but the majority of the power comes from the gas.
Do you understand how the electronic diesel injector works, it still is a pressure system, it just uses an electrical switch to open the injector. So how do you compress the gas and the diesel and control the mix.