Multiple attempts to reflash failed horribly (It's what one gets when looking at solutions from 2016...), but I found one that worked ... eventually.
Multiple fel timeouts, but it's currently writing to the PocketC.H.I.P. so ... fingers crossed?
Apparently flashing requires USB 2, and doesn't work well over USB 3.
Guess whose Linux computer only has USB 3 and C?
I also have a Rasp Pi that I've read has shaky results, but "sometimes" is better than "nope." I'll have to remember to bring it home from school today to give it a try.