That's true, and in order for the radon gas to migrate into the basement of a house it has to pass through, or more likely come from the rocks right under the house.
Common sense and even a bit of knowledge about gas diffusion tells you that even if the fracking set loose a lot of radon a mile underground, those gas molecules are not going to migrate through a mile or more of solid rock very quickly. If ever. Particularly since at the level of the fracking the flow is out of the rock and into the well.
Anybody who paid attention in science class knows that diffusion goes in the same direction as bulk flow, so any radon released by the fracking would go in the same direction as the natural gas released by fracking.
Shh, don't let the facts get in the way of the narrative. /sarc