They weren’t using RHEL. The title would be more accurate if it said “RHEL-based Linux”. They used their own build called Scientific Linux for a long time, then switch to CentOS. Then Oracle started aggressively poaching RedHat’s customers with Oracle Linux, which forced RedHat to start locking things down. One side-effect is that the life of CentOS 8 was dramatically shortened, and then RedHat killed the stable released versions of CentOS (9, 10, etc.). CentOS is now a rolling release “Stream” distribution, which nobody actually wants to use in production.

All this drama alienated many people who built their careers on RHEL-based systems, and created (either actual or perceived) instability in the RHEL ecosystem. And at the same time they started deprecating older hardware, some would say far too early. This is the “straw that broke the camel’s back”, as they said in the CERN presentation.