One issue I found was that there might be some personal quality required to get tutored effectively. Can everyone be tutored and benefit from it? For the geniuses listed, there must be an army of not so brilliant minds that we didn't hear about. How much worse off are they in a normal factory school?
Also, if you go to Oxbridge, you get a tutor several times a week. I am not sure I'm that much smarter than someone who didn't have this privilege. But of course I benefit from people thinking that tutoring is magic sauce. I wonder if anyone has checked.