That’s why I don’t let every device decide individually. I know my router (FritzBox) prioritizes the pi-hole (it’s even called “preferred” and “alternative” DNS-Server in the UI)
I have my pi-hole setup as the upstream DNS in my router, with cloudflare as a secondary DNS. That way, all my devices always use the router for DNS (since that’s what is advertised in my DHCP) and the router then uses pi-hole if it’s available, or cloudflare if it isn’t. But the individual device doesn’t get to choose between different servers.
I’m guessing it’s about documenting the assembly of safety-critical components. If some part of, let’s say an airplane fails because a bolt comes loose, the manufacturer wants to have a paper trail attached to it to prove that this specific bolt was indeed torqued to the correct spec. Connecting the wrench to the network could make this documentation much easier.
My router is called Jupiter, everything connected to it is named after a moon. Callisto, Ganymede, Thelxinoe, Kallichore are what I’m currently using.