Porting Halo 3 really isn't that simple. All Xbox 360 games were made on some weird code foundation that doesn't coincide with PC's. You would, essentially, have to recode everything for the port for it to work. So why put all that effort into basically remaking the game that's like 7 years old, when you can make its own f2p, modern version?
Also where does it say its on the H3 engine?
Presumably in some external announcement. Give it a Google if you care so much, but it sure looks like Halo 3 graphics and gameplay.
Hey, something actually decent to come out of the Halo series since 343 took over.
I'm really curious what 343 has done to make all of the fanboys so butthurt about their work, because the games have been fine. For that matter, what's (relatively) "better" about this, the fact that you won't have to pay for it?