Please be serious. Are you honestly suggesting here that Red Hat engineers first build Fedora, then forget all about it and build RHEL in an entirely different way? How do you then explain changelog messages from Fedora packages in RHEL packages?
Sure, they do put some stuff that is not in Fedora in RHEL. But they are far from being idiots, so they reuse vast majority of _already_ _tested_ code from Fedora.
"The size and expertise of the Fedora community make Fedora an ideal incubator and proving ground for features that eventually get incorporated into Red Hat Enterprise Linux. To meet the quality and reliability requirements that make Red Hat Enterprise Linux the choice for mission-critical applications, Red Hat puts Red Hat Enterprise Linux through its own set of tests and quality assurance (QA) processes that are separate and distinct from those of Fedora."
I hope you are not suggesting that Red Hat engineers "rewrite" those features for RHEL just to make it different.