Small pox was way after hunter gatherer times, so I‘m not sure what point you are making. Huge farms were a thing even in medieval times, with hundreds of animals.
My point is that factory farms aren’t a requirement for zoonotic viruses. Smallpox also predates the medieval period by thousands of years.
We also know that there are viral epidemics in animals that live in solitary animals and animals that live in groups smaller than the size of hunter gatherer tribes.
"Way after" is quite an overstatement. Smallpox is as old as agriculture. Most seem to agree that it was the transition into agrarian life that provided the necessary conditions for it to emerge, but it did so right as that transition took place.