On September 27, the Free Software Foundation (FSF) celebrates the 40th anniversary of the GNU operating system and the launch of the free software movement. Free software advocates, tinkerers, and hackers all over the world will celebrate this event, which was a turning point in the history of computing. Forty years later, GNU and free software are even more relevant. While software has become deeply ingrained into everyday life, the vast majority of users do not have full control over it.
The hurd kernel exists, and is a theoretical alternative to the Linux kernel
Right, but if they prefer we call linux “GNU/Linux”, wouldn’t they want to call their OS “GNU/Hurd” instead of just “GNU”?
Nope, because Hurd is created by the GNU project. Linux is entirely independent.
They want you to call it GNU / HURD now so they can contrast it with GNU / Linux. You are correct though that the real “GNU Operating System” includes HURD.
See Debian / HURD for example:
https://www.debian.org/ports/hurd/