Ok, so I know there has been some confusion over whether or not the redone Halo 2 maps were/are actually running on 343's new engine that is "built from the ground up for Xbox One".
This confusion originally came up when 343 anounced that the new re-imagined Halo 2 maps were running on a "native Xbox One engine" that takes advantage of the Xbox One's aditional horsepower.
This led me, and others to believe that the maps would be utilizing Halo 5's new engine, and some were dissapointed as the new map shown looked almost exactly like Halo 4 in 1080p (which led many to believe that this is how Halo 5 would/will look).
Thankfully, Frank O'Conner has cleared much of this up via Neo Gaf where he confirmed that the Re-imagined Halo 2 maps (including the one shown at E3) were not running on Halo 5's new engine (which is still being tweaked & worked on). Instead, this "native Xbox One engine" that they were talking about is simply a Tweaked version of Halo 4's engine & renderer that has been optimized for X1 & dedicated servers.
StalkerUKCG: "Frank can you tell us more about the new engine? it a Base for halo 5? Is it based on the halo 4 engine? What changed have been made? Will we find more out?"
Stinkles (Frank): "Proprietary evolution of og engines, including aspects of h4 renderer. Not 5, which is obviously still being refined as the game gets closer. Would have been crazy to split. Beta will be your first experience of 5 engine."
Final Verdict: "So basically your saying that the match we saw at E3 of the "re-imagined" Halo 2 map won't be the graphical look of the new H5 engine, and that the new H5 engine could possibly look better?"
Stinkles (Frank): "Correct"