The trick is to get the Apple TV to boot in recovery mode. This can be a bit troublesome when it’s stuck in a boot loop though.
The following steps should get it there, and get it to show up in iTunes for a restore (which will fix it in most cases):
* plug in usb-c
* plug in power
* hold menu + home until it starts blinking fast, then release
* immediately after that when the light turns off, hold menu + play for a few seconds
it may require some fiddling, i’ve Had to spend up to half an hour for really stubborn atvs, but so far succeeded every time