@stephenw10
Which means that valid USB boot drive with the good firmware etc doesn't auto boot except if the admin interrupts the boot process from the console, and types in the command 'run usbrecovery' ....
So, I was wrong. Some manual manipulations are needed then.
https://docs.netgate.com/pfsense/en/latest/solutions/sg-1100/reinstall-pfsense.html is clear about it.