Heineken mini keg. 1.3 gals

I have purchased one of these before, I have also purchased the Coors keg. The Coors mini has a pressure release valve at the top that also allows out side air in; to keep from causing a vacuum. The Heineken mini keg is completely sealed; which allows me to believe that they pump it full of Co2 to keep it pressurized. It has a pick up tube and dispenses from the top. Now for my question: if it was pumped full of Co2 wouldn't it become highly carbonated, and then lose pressure? Or is it possible that they have pumped it full of compressed air also.....????
This is just something I have been thinking about.

It actually has a small CO2 canister inside it,glued to the bottom.We cut the top off a couple of em and painted em.SWMBO uses them for flower pots.
Cheers