Well, I don't know anything about this but from what I can tell, many people who have large collections of snakes feed live. Add them up together and that's probably tens of thousands of snakes being fed live on a regular basis.
Now, I've never heard anyone who I know, or anyone on any forum talking about this disease (so either it doesn't affect them, or it is not a very large issue if it does exist).
I don't want to make this guy who was selling frozen rodents out to be a bad guy, but maybe he was telling you this so you would buy some of his frozen feeders?
Just an idea.