It's a separate partition and it's not normally mounted (so you won't see it in the mount or df output), so you can't corrupt it by overfilling any of the mounted partitions. It's not like a hidden directory or something on one of the normally mounted partitions, it's a separate one.
So yes,
it could house the image on the module and a reboot ota could reset to start without fear of the ota ever becoming corrupted from operational usage
and I've actually did something like that a couple of times when testing new packages. I've written the image to the OTA partition, then ran "reboot ota", it reflashed itself and I've got a fresh system. Then I've tested several packages and during the process I overfilled the rootfs partition, so I just ran "reboot ota" and it was fine again. And a couple of similar cases, so yes, it works in this scenario.