A lot of the halo lore wasn't fully established at the time of halo CE so there are a few discrepancies. Johnson a few marines managed to take a pelican and fly out into space. Chief met up with them and brought them on board his longsaber. He also rescued Linda (the other Spartan onboard the pillar of autumn, far too injured to fight), who's cryopod had been ejected into space before landing on the ring. Together, they launched and assault on the surviving covenant ship "ascendant justice", managed to take control and used it to escape the ring before other ships could notice and before a prophet and his very large escort fleet could arrive. They than jumped back to reach, now entirely glassed, and met up with Dr. Halsey and the remaining Spartan IIs protecting her.