In fact, it almost sounds like copyright—or, would have, if not for disney.

Yes, I think that's the objective for the licence. Reading slightly more into the product (Halo 2), they would benefit from more adoption by others as it could facilitate cross-chain interoperation.

The license model gives everyone equal access to the code, and equal rights to improve Halo commercially (make money?) with the promise that they subsequently open-source their improvements after 12 months.

This license model fits this product quite well.

That was my impression too. However, where a patent expires, I couldn't see when the source code had to be released by. Seems a bit problematic if that is the case. I also don't see a substantial difference to just releasing the source code at a later date under a open source license when the copyright holder chooses to.