Erasing to send to Apple for refund bricked my iPad

I followed the instructions to prep my old iPad before returning to Apple as a trade-in toward a new iPad. It worked perfectly and was in great shape. I signed out of iCloud, deregistered iMessage, removed the device from Find My list, etc. and then as per the instructions tapped on General > Transfer or Reset [device] > Erase All Content and Settings. That done, the iPad tried to reboot, but failed. I didn't think much of it and returned it to Apple. They sent back an email saying the trade-in was worth $0 since I hadn't turned off Activation Lock. But to do that, I have to find the device in my Devices list in Find My and of course it isn't there. Likewise, if Activation Lock *was* the problem, it should boot to the "device is locked" screen. So I don't think. this is an Activation Lock problem -- AFAIK it should have been turned off when it was removed from the Find My list.

Apple did offer to send back the device, but sure enough, it just tries to boot over and over. Hard reset (Home and Side buttons) doesn't help. As far as I can tell, Erase All Content and Settings made the device impossible to boot OR restore. Any help would be very welcome.