> I’m arguing that exactly-once delivery is possible if the message receipt happens in a single place, but maybe others don’t see that as a distributed system at all.

I think that only holds if the sender is also in the same place. Otherwise there's the very real chance of a message getting lost, turning your system from exactly-once into at-most-once. At this point your sender, consumer, and messaging systems are one system, so it's probably reasonable to question if that's a distributed system.