The events of Marathon is all a dream.That's why Infinity's chapters are titled Despair, Rage, and Envy, and the final level being Aye Mak Sicur.
Infinity's ending shown the Security Officer's acceptanceMixing this in with Durandal's optimistic ideals of humanity, all the torn bone and flesh hosted by machine were Dissected, and all that was left are strands of artificial muscle and skin tissue still necessary. The apparent disfigurement in his fingers are damage from an explosion.
The Security Officer will return in Destiny, which is a distant sequel to MarathonDon't forget that "Destiny" was the name Durandal gave the officer as the universe closed. Or those leaked documents that claimed Bungie was working on a Marathon sequel. And perhaps the Traveler is Durandal in his Jjaro dreadnought.