Not many people I have tired this I bet but out of curiousity I tried blowing and sucking through the radiator overflow tube, well as you do. (cortinal radiator)

What I foundto my surprise is that the overflow tube is not sealed, I expected it to be so started to wonder if the radiator cap is duff.

Thinking more about it on the journey to work, it should be ok as the cap seals off the radiator so it does not matter as the tube should be above the level of the seal so vented to air. When the water expands and pushes the sael open, water can flow into the neck and into the tube. When water contracts, there will be water in the neck and the inner valve opens to draw water back in. So it seems to me all is well with the cap.

Now, not everybody would admit to getting THAT intimate with their car.

You ar spot on, that is how a system without an expansion tank is meant to work. The cap should have two seals, spring loaded one to pressurise the system, and one which seals on to the very top of the radiator neck so that the water from the bottle is drawn back in and not just air from atmosphere or you end up pumping out the water and replacing it with air.