The blog post might not be very clear on this, but I think the research mentioned basically shows that some chlidren are very strongly convinced that they "don't have a mathematical mind" or something along those lines, and believe that other people grasp mathematics without effort, which produces some sort of psychological barrier that stumps their abilities. It's not really about denying genetic variation in intelligence, more about getting the children to understand that even very able people have to struggle quite a bit to learn hard things.
A sort of mirror image phenomenon mentioned in related studies is that kids who strongly believe themselves to be gifted may be unwilling to try things that are hard, because their self-image is tied up with things being easy for them.
