Oh, I see. The instructions for actually using the installer[1] -- which aren't mentioned in the repo containing the installer images[2], much less linked from it -- tells me that, during the last step of the install process, I have to switch to another virtual terminal and hand-edit the configuration. And if you don't do that, you get a probably-unusable install.
Awesome, great.
[1]: https://systemcrafters.net/craft-your-system-with-guix/full-system-install/
[2]: https://github.com/SystemCrafters/guix-installer