Ok I think the installer I used may be a bit out-of-date. I found this thread:
Seems to me I should try:
rpm-ostree rebase ostree-image-signed:docker://ghcr.io/ublue-os/bluefin:latest
or even (since my laptop is quite new)
rpm-ostree rebase ostree-image-signed:docker://ghcr.io/ublue-os/bluefin:40