I wasn't able to get it to work through the WebUSB installer under Fedora, but it was fairly straightforward from the command-line. The official command-line guide won't be of help, in that it will get stuck on the same underlying issues as the WebUSB install, and it is all Ubuntu.
Here's what I did (to the best of my recollection/reconstruction)
Follow the official instructions up to the point of having OEM unlocked phone booted into bootloader and ready to connect to Fedora PC.
sudo dnf install android-tools
/connect phone via USB/
/see if fastboot sees phone/
sudo fastboot devices
sudo fastboot flashing unlock
download the appropriate file from https://grapheneos.org/releases and unzip
cd into the directory and
sudo fastboot flashing lock