The mention of UEFI in this context likely means they are thinking of a deletion recursing through sysfs and by extension deleting all visible UEFI variables which, in some firmware editions and versions, causes it not to be able to get through post or into the setup menu.

I vaguely recall this and the general issue was very bad firmware design, but it was possible to make it impossible to even reinstall a system. If you were industrious in windows you could have done the same thing, so malware under windows could also brick such platforms.

Of course rm has more safeguards on it so you have to pass more flags and really really be asking it to try to screw things up.