You are right. I bought my son a VEX robot and I took a look at their pneumatic system for it and all they use is a reservoir. I figured it must be useable.I think if you were using an efficient prop like a TCT or a simple pop up, it might last you for some time , but any prop that wastes air or has multiple movements would likely crush that small tank in no time. they said you can get 500 brads out of a tank. Seems like a decent amount, but then again you are only using a very small burst of air.
It's too bad you couldn't refill that at home like one of those 5 gallon air storage tanks.