That sounds very strange, considering the way they handled that in previous Halo games. Bungie has always used campaign assets for MP maps, but they never used exactly the same geometry from Campaign to create MP maps. An obvious example is Construct, clearly inspired by the end of The Ark level, but still quite different. I suppose that's what they meant when they were explaining it.
Anyway, if they really are using the same geometry for Campaign and MP (hope not) the only way I think they can make it work is creating the MP maps first, then addind them as sections of the Campaign. The other way around would be just silly.
Bye