Hm, ok if I hold down power AND volume down I can get into a mode which has the following:
* system info
* show barcode
* reboot to bootloader
* reboot to download mode
* reboot
* power down
`adb devices` still doesn't list anything though, so the flash script fails.
This must be download mode, since choosing "reboot to download mode" comes back to this screen.
Choose "reboot to bootloader" gets me to bootloader screen, which has more choices, like "reboot to recovery".
`adb devices` in bootload mode still doesn't list anything, so the flash script fails.
Going into recovery mode, more interesting choices:
* Apply update from ADB
* Apply update from SD card
* Wipe data/factory reset
* Wipe cache partition
* Run graphics test
* Run locale test
Choosing "Apply update from ADB" sounded promising, so chose that, and sure enough the devices shows up in adb device list now, but with the word "sideload" next to it, and no other adb commands work.
Feels closer anyway. I wonder if maybe there's a way to sideload the update ROM?
At the least, the support document should be updated to say to hold down power and volume down, not just volume down.