This mod has the Anniversary graphics disabled, because a lot of the additions are either cut content or made from scratch/ported from other mods. This means that they don't have Anniversary models, which crashes the game when trying to load the new graphics.

Also, yes, the Halo 3 mod is using the original graphics (Halo 3 never had an Anniversary remaster), and they probably are better than what you remembered.

Curiosity: Kashiera (I probably misspelled) is recreating Halo CE on Halo 3 (ODST?)'s engine, and it looks better than the Anniversary version lol. The mod is called Halo 3: Combat Evolded, you can check it out here on the Workshop.