You are absolutely right, there is the issue of pump vacuum lock. On the return stroke, if the piston seal creates a perfect vacuum, the piston is not going to pull out clean and can lockup.
The V-shaped seal seen on all modern multi-pump pneumatics is a great solution. The V-shape will be much less efficient sealing on the return stroke.
But, obviously, the v-shaped seal is not the only solution. After all, they didn't have v-shaped plastic seals back in the 1700s, yet they still produced workable hand pumps. The pump on my London air cane is a solid rod perfectly fitted to the compression cylinder. Unlike the modern pneumatic air pump where the pump works at any rate of pumping, with these old pumps it took a "QUICK" stroke. For instance, I never pump my London air cane, there has to be some young kid hanging around looking for a real workout. The need for a quick stroke is that the pump does not seal as well as a modern piston head so that a slow pump will not create high pressure because air leaks away. A fast pump is needed to create that burst of high pressure before it leaks away.
My guess at why this design apparently works and another design didn't is that with this design the piston is very long and well fitted to the pump cylinder. The o-ring is narrow. It's the piston cylinder itself, not so much the o-ring, that is creating the seal. This design looks like it is a throw back to the 1800s.
Another design with a short piston cylinder that relies more on the o-ring to provide the seal would probably be more prone to vacuum lock.
In any case, without a vacuum release valve (something pretty much nonexistent in air guns), it's always a balance between sealing on the high pressure power stroke and vacuum lock on the return stroke. The modern piston seals provide a great balance, but, there are alternatives.