They ought to have said Gnu/Linux, as per their own propaganda!
I'm guessing if the word "Hurd" doesn't appear anywhere on the page, it's still Linux.
Linux is just the kernel. GNU as a name for the system is okay, while Linux is arguably not. GNU/Linux is just a compromise for those who insist on naming Linux. Also, from the projects website, we have
"GuixSD is a distribution of the GNU operating system centered on the GNU Guix package manager. It uses the Linux-libre kernel, and support for the Hurd is being worked on."
The site should be more direct about whether or not it is Linux.
I don't have time to wade through reams of words to work out what something is. Just tell me.
It's not clear at all why this matters or is better than other operating systems. If this is a competitor to Linux then the first thing they need to address is "why should you care?", if it is Linux, then the first thing it should adress is "Why you should use this over other distros"