Earlier quoted context omitted.
I have a backup NAS that's normally powered off, but it's scheduled to turn on, perform backup, shut down. It doesn't wake on LAN and there should be no way of knowing it exists outside of checking DHCP static addresses reservations - and now that I mention it, maybe I should remove it from there too. This minimises the size of the window, and network-snoopable information, required to compromise this set of backups.
I think, perhaps ignorantly, That may prevent some human being or intelligent agent specifically targeting your nas. I don't think it would help against situations where your primary system is being encrypted for a while, and thus your backups eventually get overwritten with bad stuff.
I use rsync's --link-dest to get Time Machine-like backups so they can't be overwritten through the backup system itself. I don't take the "shut it down" precautions with this system the other person does, though I do have a separate in-place one that's strictly manual (as in I have to physically plug it in to update).