@NRP I'm not certain but I think they might have MP Playlists that are split out by gametype and game-engine, such as BTB H4 or BTB H3. I haven't gone into the MP side yet.
The reason they kept the maps within each specific engine was to pacify the long-time MP players who have memorized all the individual quirks of the original Maps on the original game Engine. If they ported them all to H4 engine, they would have complaints about how the maps no longer play the same because quirk XYZ isn't there.
343i can't win either way. I haven't picked this up yet, but definitely will for the SP alone. Currently, I still need to finish Halo Wars of my backlog. I wish they would release a Game of the Year edition that would include the other ports they are working on.