Solved - evidently the version of the firmware on github contains the error I read about in a different thread (forget which one) where basically you have to hold the prog key as you plug in the keyboard. I did that, then pressed “enter” on a different keyboard and the firmware flashing worked.
Is there a fix planned for this issue? It seems to have been introduced by whatever’s in the repo as of today, as I did not need to use this workaround when I did my test flash of the firmware a couple hours ago. Not a huge deal since I have an easy workaround, but still seems like unintended behavior.