Well, when you consider that Michael Jordan obsessed about his height before he was tall, there's a good chance that he felt that it was at least partly responsible for his skill.
In actuality, it did play a significant role. If Michael Jordan had the same level of skill, and was 5'9", he would have been above average(for an NBA player) at best.
I think reverse Dunning-Kruger is much more likely in Magnus's case.
"...the most competitive individual...Always felt like somebody else was going to outwork him so he wanted to out work them." - A trainer of Jordan's. (Video for reference: http://youtu.be/w39-_rauFSk)
just like basketball is not purely about body size and athleticism, chess is not purely about iq. And it is kind of idiotic that iq is the thing that people think must be the one thing that makes good chess players good.
after a certain point, iq probably doesn't matter. motivation, concentration, competitiveness, determination, experience, preparation, risk-taking, creativity, pleasure when thinking about chess... i really don't understand why iq is the thing that's talked about so much.
magnus clearly understands that some people will neglect chess for something else. The implication is that there are geniuses out there today doing other things instead of moving figurines over a grid... that's a very humble and wise world view.