There's a little more to it than that. The announcement doesn't cover the history CentOS has had with RHEL, but when CentOS people found bugs or made improvements, they would pass the info back to RHEL. It makes sense for CentOS because when they make improvements, they can hope that in the next release, they can just reuse RHEL work rather than having to apply the patches each time. It made sense for RHEL because they were getting a better product to offer their customers than they would have without the CentOS contributions, and by integrating the work of their biggest potential competitor, they decrease the incentive to move to somebody who has patches and improvements they don't.
It's rare to read about "synergy" between companies that actually makes sense, but RHEL and CentOS have benefitted from each others' work. The more RHEL helped CentOS, the better RHEL software was. The more CentOS helped RHEL, the better CentOS software was. This move to actually formalize their relationship makes sense for both of them.