Where did you read about the combustion system? I'd love to get more details on this marker. It would certainly explain how they surmounted the combustion issues. I couldn't beat my problem with a reasonably sized chamber. I had to go with bigass can about the same size in the PEP if I were to keep things at a starting pressure of 1atm. I can see how they could promise consistent performance with temperature if they went with combustion.
I would think that they would just go with direct drive using the direct exhaust instead of driving a piston. The thermal impulse would be pretty short so I don't think the pellet would be damaged. Clanging around a piston works fine with a nail driver because the speeds aren't that high, but with projectile speeds, you end up putting a lot of energy into moving heavy metal parts instead of the projectile. Not very efficient.
I have reservations threading a tank rigidly onto a gun like the PEP. I've ripped off the fittings off of empty propane tanks (intentionally to examine the valve design). I chuck a propane adaptor in my lathe and thread the tank on. I don't have to push too hard on the tank to yield the metal around the valve fitting. A couple flexings and I can rip the fitting right off. A bottle threaded onto a rigid rifle could have the fitting ripped if a player falls on their gun and bends at the root of the fitting. An entire bottle of propane venting in one go is a fair bit of propane.
Want nearly free GBB gas?