So I removed /etc/systemd/system/systemd-logind.service.d/override.conf and /etc/systemd/system/systemd-hibernate.service.d/override.conf, rebooted, cleaned up journalctl, ran sudo systemctl hibernate. There was no warning about swap size 
But it didn’t work: screen went black for 1 sec, then login screen. I notice the following messages in the log:
Sleep mode "disk" is supported by the kernel.
Disk sleep mode "platform" is supported by the kernel.
/dev/zram0: ignoring zram swap
/var/swap/swapfile: detection of swap file offset on Btrfs is not supported
/var/swap/swapfile: device matches configured resume settings.
Hibernation will attempt to use swap entry with path: /var/swap/swapfile, device: 259:3, offset: 8332544, priority: 0
Enough swap for hibernation, Active(anon)=1632748 kB, size=20971516 kB, used=0 kB, threshold=98%
But also lots of these (with different destinations), not sure if related?
Got message type=signal sender=:1.16 destination=n/a path=/org/freedesktop/systemd1 interface=org.freedesktop.systemd1.Manager member=UnitRemoved cookie=2662 reply_cookie=0 signature=so error-name=n/a error-message=n/a
This is the full output of sudo journalctl -u systemd-logind.service from the moment I run the hibernation command, with debug enabled:
This is readable if you hit download and open in a text editor with word wrap disabled.
I can’t find anything specific in there, any moment when it fails to hibernate.
FYI: In my bios (HP Spectre x360, i5-1135G7), TPM STATE is enabled, TPM DEVICE is available.
Should I disable TPM STATE? Or would that require me to re-install Silverblue?
I barely use my laptop outside of my own house so I am not sure if TPM is really important for security in my case.