The jist of the argument is that no DB gives 100% availability. That makes no sense. What if my client can't talk to anything? I'd still have to return a value.

The jist of the argument is that no DB gives 100% availability. That makes no sense. What if my client can't talk to anything? I'd still have to return a value. So instead we talk about high availability. All sorts of things cause downtime besides network partitions. If the network is reliable enough, then you can still achieve high availability.