That’s what I missed! I added the non guix channel, but I didn’t do the last two crucial steps specified in the nonguix readme:
Guix System will only use the substitution server after it has been
reconfigured. The substitution server will therefore by default not
be used the first time you run guix system reconfigure after adding
the substitution server. It is therefore recommended to explicitly
specify the use of the substitution server the first time
you reconfigure your system:
sudo guix archive --authorize < signing-key.pub
sudo guix system reconfigure /etc/config.scm --substitute-urls='https://ci.guix.gnu.org https://bordeaux.guix.gnu.org https://substitutes.nonguix.org'
Once I did the steps specified above I was able to reconfigure! Thanks for the help!