That was my first thought as well. I can see how air relatively close to the surface could geyser out, but I'd guess that deeper air would likely be trapped. Given the high pressure and density farther down, compressed blueberries would probably behave like the rock that traps water and methane in the Earth's crust. Because of the trapped air, maybe the planet would shrink a little less than the author predicts. (Of course it will shrink a lot, because the air compresses, but maybe just a little less than predicted.)