Wouldn't that situation be fairly common for backyard chickens? I feel like most people who keep chickens in their backyard aren't going to have contact with other chickens.
The exception would be a neighborhood/community where a lot of people have backyard chickens. But even then, wouldn't the chance of infection still be low?
It's not chicken flu, it's avian flu. You have to control for contact with all birds. I have a neighbor with chickens. They aren't always cooped up so they often mix with the wild pigeons and mourning doves. Even still, they just have simple cages that can be pecked/pooped through.
Completely enclosed runs and coops are very much the exception to the rule. Most backyard chickens are just fenced off (they can't really fly after all), meaning they're exposed to other birds.
The risk it outside wild birds. If you have feed on the ground and wild birds are also eating it (and pooping on it), your Chickens could be infected.