Pre-configured installer images point to x86_64 images.. which is probably not what you want on other archs. Not sure if the default installer image list is something configured in fedora or whether this is an upstream gnome-boxes bug.
Most of the data should be coming from https://gitlab.com/libosinfo/osinfo-db/-/tree/master/data/os/fedoraproject.org From a quick look, I could not see anything off for aarch64, so maybe gnome-boxes also has some logic around that.
Yes, Boxes really hardcodes x86_64 in this case. I will work on fixing that. But when it comes to virtualizing aarch64, Boxes needs some more work to be able to create the VM with the proper domain capabilities (https://gitlab.gnome.org/GNOME/gnome-boxes/-/blob/main/src/vm-configurator.vala#L49). I will work on supporting ARM and I hope to ship that in the next gnome boxes stable version, 43.
I have an upstream PR for this one here: https://gitlab.gnome.org/GNOME/gnome-boxes/-/merge_requests/619 that needs some attention but otherwise enables aarch64 support, and just recently one of the patches it depends on was merged.