They could have broken out at any time, but Katooni was surprised that Petro was able to do it. All the younglings were under the impression that they'd be trapped in the cave if they failed to get out in time. For Petro, finding the crystal would allow him to get out and not get sent back in, resulting in a pass on the Gathering test. I see your point that this was just to save himself... however, it's a bit of a moot point since he reneged on this decision a few seconds later. He was willing to risk failure to save a comrade. Edit: by "save himself" I mean "get his crystal".