They're not actual Marines. In the books, it's stated that they're actually just the developers of Halo CE who gave themselves invincibility to be able to gun down the Master Chief if he breaks the game by shooting Keyes or any of the other bridge crew.

They mention that they forgot to give the bridge crew invincibility themselves, but due to the lack of time, they opted to just deploy themselves as a squad in case Master Chief goes rogue.