It's the opposite. Plenty of subsystems in the RHEL 8.3 kernel are basically on par with upstream 5.5 or so, as almost all the patches are backported. The source code is really the same to a large extent, and therefore security fixes apply straightforwardly.
Plus, there are changes (especially around memory management or scheduling) that are fiendishly hard to do regression testing on, so they are backported more selectively.
The upstream for most other packages generally move much more slowly than the kernel. The fast ones (e.g. X11, systemd, QEMU) are typically rebased every other update or so (meaning, roughly once a year).
It also helps that Red Hat employs a lot of core developers for those fast moving packages. :)
