To clear some things up: Halo Anniversary is the halo 1 engine with improved graphics, it uses the same physics and balance as halo 1.

There is no Halo HD multiplayer, the multiplayer you are seeing is the MAP PACK of halo 1 based maps for Halo Reach that will be included with the game. It is unclear at this time if the maps will come with the full reach multiplayer or just an installer for the pack.