Coming back to this after another 18 months or so: I'm now of the view that Red Hat's own core operating system business model is actually of the "delayed open source" variety, since the definition of the core package set for RHEL & CentOS is entirely up to Red Hat, with the source code being delivered to CentOS at the same time as a new version of RHEL becomes generally available to subscribers. Thus, RHEL is to CentOS as Android is to AOSP. The difference is that Android doesn't have any equivalent to Fedora as an upstream integration project that creates a pool of public components, for Google's to filter and create the base Android platform.
Third party contributors getting a change accepted into Fedora isn't a guarantee of seeing that same change accepted into mainline RHEL (and hence CentOS), it just dramatically lowers the barriers to having that happen.