Okay, it’s possible I’m misparsing the manual:
You can put the keyboard into programming mode by pressing the X key when the OLED menu is opened. […] So only do this when the flashing script asks you to do so. You can recover from a stuck keyboard by toggling Pocket Reform’s Standby Power Switch off and on.
Am I reading that right, that I run picotool, and during that flashing process, I’ll be asked to put the keyboard into programming mode? If so, I get the above error before I’m ever prompted. If I need to get the keyboard into programming mode before running picotool, then this is straight user error on my part (possibly with an assist from the docs).