It all depends on how much each fuel costs.
Around here diesel goes about $4.09/gal ($29.21/MMBTU)
Propane goes about $2.79/gal ($29.36/MMBTU)
CNG goes for around $1.50/gge ($12.50/MMBTU)
That's retail at a filling station set up for fueling motor vehicles.
CNG has an advantage because its not taxed. That will certainly change, at least in commercial filling stations.
Propane and CNG are just auxiliary fuel. By themselves they'd never run a diesel as their cetane rating is too low. In a diesel you used the No.2 injector as a spark plug to blowtorch the aux fuel into burning.
The guy is not being honest with us (or himself) because he is not telling us the propane flow rate.
CNG requires tanks and piping rated for either 3,000 or 3,600 psig. Don't know about propane. To make CNG you need a compressor that looks a lot like a scuba air compressor but with a bigger aftercooler.
There is (or was) a home CNG compressor called a PHYLL that would take a day or two to fill a 10 gallon equivalent CNG tank. It needed 240 V power and a regular residential gas service. A big (more commercial style) Ingersoll-Rand compressor needs a 480V 3 phase service and 50 psi
gas service. It will fill up a 10 gallon equivalent tank in about 12 hours.
If I had an economical source of either gaseous fuel I'd use it in a spark-ignition engine. It would be simpler.