Thanks for the link to Jeff Geerling’s blog.
I did the same thing to my reTerminal when I first powered it on: tried to do the update before I saw the warning not to.
I used Jeff’s procedure to get usbboot to mount the eMMC and am now waiting for rpi-imager to flash /image_2021-09-14-Raspbian-reTerminal-arm64.zip
I’m excited to get into development on the reTerminal.