I'm going to have to disagree with him. If I need to break compatibility on a single resource, I don't want to mirror every API endpoint (except that one) to a new hostname. That becomes continually more and more complex and hard to keep sense of as changes progress.

The hostname thing might work for huge, world-changing re-writes, but for small, incremental changes, I think something like what GitHub does w/ the Accept header is far more appropriate.