Well, first and foremost, it was clearly a way-kewl marketing gimmick to help promote the final installment of the series. Very well done and thought out. A lot of effort went into that.
Besides, what with New Mombassa being destroyed by the in-atmosphere hyperspace jump by the Prophet of Regret's capital ship in the second game, who's to say what happened exactly in the final moments of that conflict.
Unless, of course, I'm getting the different Earth cities mixed up, and it was some other city that was destroyed in HALO 2. It's been known to happen before; my memory ain't what it used to be, lol.
Clearly, in the game cinematics, the Master Chief managed to escape with the
In Amber Clad, so if you wanted to relate it back to the storyline, the diorama scene depicted in the ad could've taken place a little while prior to that.
And, don't forget... the Master Chief wasn't "dead" in that final shot. Though the Brute holding him clearly thought they had a victory there, he
did raise his head and the Plasma Grenade in his hand glowed.
He may have been momentarily knocked unconscious, but he wasn't dead.
No doubt the Master Chief dealt that Brute one helluva @$$-kicking, lol, and then managed to get the hell out of there.
If you really wanted to put it in a plausible context, that's one way to look at it, but I think it's more a matter of personal interpretation than what actually occured. It allows one to use thier imagination in what happened there, and further get them psyched about the final installment of the series.
That would be my take on that scene, anyway...