Ok - for all you doubtors, I took 6 hydrated pellets and lightly formed them together to minimize air between each of the 6 pellets. I squeezed the enlarged pellet under water. Low and behold at least 5 or 6 small bubbles came out of the larger pellet. Now I suppose you want to see the video as proof.
No video as proof needed. I believe you. Perhaps you are right that the mechanism that causes the pellet to sink is removing the air.