I think its pretty clear based on the way Bungie keeps talking about the time spent on ODST versus Reach that they knew exactly where one stands in relation to the other. Wasn't the whole thing supposed to be DLC in the first place?
I'm sure Microsoft stepped in and changed that quickly once they saw the relative poor sales of the GTA DLC (which is unfortunate since whatever you think about those games, those are the best quality level, and content level DLC this gen imo) and they knew they could milk ODST into something.
As far as them talking it up, once your name is on it, that's simply what any dev is going to do. They can't run away from something that has their name on it.