But many of Halo's traditional maps don't follow any kind of reasonable campaign structure, which makes many of them unlikely (again, not definitely) to make an appearance.

Honestly, it seems very akin to how Infinity Ward approached Modern Warfare 2's multiplayer maps. Not sure if it was just bad wording on the interviewer's part, but here's what Gamespot said:
Lehto explained that all of the multiplayer maps are taken directly from the single-player campaign, but in the case of Power House, the multiplayer version takes place during the daytime whereas the single-player incarnation is a nighttime affair.