Probably I'm hung up on "Firely's engines are actually self-pressurized". If they actually mean that, it's some sort of novel system. If they actually mean self-pressurizing tanks then that's a bit more normal--though I still would want to know if it's a blowdown system or if they are using some sort of gas generator (perhaps using heat from the cooling system) for regulated pressure. Either one requires a reasonably strong tank, but maybe that's not a problem for a small rocket due to minimum gauge issues.
It's a nice cheap way to go, but still somewhat unusual in launch systems. More common in upper stages and manuevering thrusters.