Yes, true 'gas' shocks just keep the oil under pressure, it's the shims & valving that create the shock absorber action. Some shocks separate the gas from the oil with a bladder or floating piston (DeCarbon), so that the oil does not become emulsified, plus you can run the shocks either way up.
These 'gas bag' shocks just allow the shocks to be sealed without any air or gas floating around in there to aerate the oil, when reassembling the shock, the oil is filled to the top of the shock body, with the rod extended, once sealed, any oil displaced by the insertion of the rod is absorbed by the compression of the gas bag, or closed cell foam. If the valving is correct, this design is ample for short vmx events.
I guess on a true 'Gas' shock, 150psi of pressure on the oil will reduce the chance of aeration more than not having that pressure.