> I guess I'm confused on what the difference between "being the most popular Linux DE" and "being the default DE of the most popular Linux distros" is.
You put the things in quotation marks but I do not see these phrases in the thing to which you're commenting.
KDE is roughly a year older than GNOME.
Snag: KDE was built in C++ using the semi-proprietary (dual-licensed) Qt. Red Hat refused to bundle Qt. Instead, it was a primary sponsor of GNOME, written in plain old C not C++ and using the GIMP's Gtk instead of Qt.
This fostered the development of Mandrake: Red Hat Linux with built in KDE.
In the late 1990s and the noughties, KDE was the default desktop of most leading Linux distros: SUSE Linux Pro, Mandrake, Corel LinuxOS, Caldera OpenLinux, etc. Most of them cost money.
In 2003, Novell bought SUSE and GNOME developer Ximian and merged them, and SUSE started to become a GNOME distro.
Then in 2004 along came Ubuntu: an easy desktop distro that was entirely free of charge. It came with GNOME 2.
Around the same time, Red Hat discontinued its free Red Hat Linux and replaced it with the paid-for Red Hat Enterprise Linux and the free, unsupported Fedora Core. Fedora also used GNOME 2.
GNOME became the default desktop of most Linuxes. Ubuntu, SUSE, Fedora, RHEL, CentOS, Debian, even OpenSolaris, you got GNOME, possibly unless you asked for something else.
KDE became an alternative choice. It still is. A bunch of smaller community distros default to KDE, including PC LinuxOS, OpenMandriva, Mageia... but the bigger players all default to GNOME.
Many of the developers of GNOME still work for Red Hat today, over 25 years on. They are on the same teams as the developers of RHEL and Fedora. This is a good reason for GNOME OS to use a Fedora basis.