I followed instruction https://docs.fedoraproject.org/en-US/fedora-silverblue/installation-dual-boot/
After a few quirks I can boot on Silverblue, but I have no users.
At first it repeatedly started Initial-Setup (very annoying). But I did this:
Add “InitialSetupEnable=False” under [daemon] in /etc/gdm/custom.conf
Now I can confirm, I cannot login as a user, from Gnome or a console (Ctrl-Alt-F2 …) is working.
(Of course, I cannot remember the root password…)