The image has GRUB to boot Linux-Libre, which in turn runs directly Guile and an init script in Scheme from its initrd. To put it differently, it boots Guile and its device driver environment known as Linux-Libre. :-)
In addition to being fun, it actually has a practical impact. First, Guix now has infrastructure to create bootable GNU/Linux-Libre QEMU images.
Second, using Guile in the initrd proves to be very convenient: all the expressivity and power of Guile and Scheme, without the complexity and code duplication associated with specific tool suites like klibc, Ash, and BusyBox.
Happy birthday Guile 2.0, and happy hacking!