Claiming that it is a fact, doesn't make it so. Even if it true, it is different from your original claim that most distributions are just Debian with a different package selection. It is a world view of Linux distributions that revolves around packaging format, which is quite obsolete. SUSE for example is originally derived from Slackware and Mandriva while originally a Red Hat Linux derivative has diverge enough from its roots. While Fedora and CentOS share a packaging format, they are very different distributions. The interesting differences between distributions have nothing to do with a package format which is pretty much a archive with additional metadata and RPM/Deb is similar enough that this isn't worth pointing out anymore. Especially in the enterprise world, things like certification and lifecycle (RHEL is 7 to 10 years) are far more relevant.