It's *supposed* to restart the device.
Granted, it's also supposed to swap to the updater extremely early during the boot process, too

.
Make sure you've identified your device correctly?
EDIT: Given your S/N, that should indeed be a PW2 ("6th gen" in the US, as Jon pointed out).
Fun fact: while your link offered 5.12.1,
this one offers 5.12.2.