Keep in mind that the talk in question is talking about the web site (and some other stuff) going into production, and as is mentioned in the talk, it is done more than once a week, and the whole shebang can be pushed in some number of minutes (I forget the amount mentioned).

Back-end services have their own release schedules and times, and obviously are made to be highly backward compatible so that they don't need to be done in lock-step with the front-end.

I think you're right about the "diamond dependency" model, but I think the merge-up and merge-down in Vista had more to do with having multiple independent branches in flight at the same time.