Ordinarily, when aluminum reacts with air, a coating of aluminum oxide (aluminum rust) forms on the outside. This is why aluminum goes from being really mirror-shiney to dull if left unprotected.
In the article, the gallium prevents the coating from forming and the pellets have a lot of surface area compared to their weight and volume. So the alumina sloughs away instead of coating the pellets, constantly exposing new aluminum to the water where it reacts to pull the oxygen away and leave the gaseous H2 behind.
Thanks for taking the trouble to reply, but that wasn’t my doubt. Someone mentioned earlier that [pure] molten aluminium is an explosive when it reacts with oxygen.