Reaction of Halo Graphic Novel

In the story Breaking Quarantine, I was confused to where Sargent Johnson went after making it to the entrance of the complex. The Flood was right behind him but he used a fragment grenade. The only thing is that he had no weapon to fight anything he comes across when getting away. In the game, they explain his escape like in the book but they stop at the end of Breaking Quarantine. He escapes the ring before it blows up in a pelican drop ship but the question is how he does it from the complex. They could have gone deeper into the story cause the Flood do spread to all parts of the ring world. Johnson was one of the only Marines to survive the harsh battles of Halo and make it to the end. The other Marines to survive had no background so it would have helped if they continued Breaking Quarantine to the point of the Marines grouping up, the Flood taking over, and the Gravemind being created on Halo even though it was destroyed in the blast.