I burned the .iso using dd as said in the instructions.
I tried also a second time with Balena Etcher, and it game me a warning saying that the .iso lacked a boot partition so it was not bootable. When I tried to boot into it anyway I got the same error as in the picture.
(Missed this initially as we posted at the same time.)
I recommend doing it from the EC update ISO live boot (which is offline).
EC updates complete by cutting power to the system, it can’t do a clean shutoff since the EC firmware that booted the system has now been replaced.
It probably would work from Qubes dom0 (I have not tested this), but the risk of disk corruption is greater as the ectool can’t sync filesystems for any Qubes before shutting off the system, I believe.