I'm sure he doesn't mean it as though he's really excited about Civ 5 and doesn't care about Civ 4... but more that the idea is a huge enough undertaking that it couldn't be done in the next Civ. More long term.
All things considered, though, it's probably best to actually move towards LESS detail with combat rather than MORE detail. But be that as it may, this doesn't have to mean less complexity. If you spend less time figuring out which cities build units or which squares they move through to get to their destination, you can add new concepts like "do you approach quickly/aggressively, or cautiously/accurately" or cutting supply lines or stuff in that vein.
Less the drudgery of a fieldmarshall pointing units towards their destination, and more the general telling them an overall strategy.