Any unpassed test should be remedied by closing, or your attorney would not let it go thru. Sometimes, your attorney may withhold thousands from sellers to make sure the problem corrected. I think it is the seller's responsibility as your attorney probably will tell you the same thing.
On the other hand, I never heard of any Radon in Greater Edison area, although theoretically, it can be any where. Mostly I heard of Radon is Nothern Jersey.
I am wondering how many years the seller lived in that house? If they live there for many years without any problem, there is reason they do not believe there is Radon. On the other hand, if the test from professional, it should be taken care of.