Biggest problem is for the results to be accurate or fair everyone has to take a different test. I.e. you'd give a physicist a different test to a, oh say street cleaner of the same age. And that physicist would have to answer what would look like harder questions right (in some sections) to get the same score. Now you have to do the same sort of tuning by where someone lives and grows up etc. So you wouldn't give a 14 year old from a 1st world country the same test as a kid growing up in the majority of sub-Saharan Africa. Or even adults from the same places.
Now when it comes to this study it wouldn't surprise me if they gave the same test to everyone. Those using opera are paying so probably better off (on average) than those using anything else, so they probably also have good educations, come from richer countries etc. Where a lot of those still using IE6 are probably using it because that's all the machines they can afford to access can handle. So you'll have a lot of 1st world professionals taking the same test as a lot of 3rd world labourers, where these populations will not have an even representation across browsers.
EDIT: Not saying IE users aint idiots though.