I'm not too surprised by this. I have often thought that Windows and IE users are less intelligent on average than users of other operating systems and browsers.
Not because only stupid people would use them. Rather, stupid people will go for the "default" option. Windows is the "default" option on most PCs on the high street (other than Apple, which is a more expensive option) and IE is the default browser for Windows. I suspect that if Firefox had been the default option on Windows for a good few years a similar study would show that Firefox users are less intelligent on average.
