That's an interesting study. I do think the milestones they are looking at are way too limited, though. Most parents (assuming they are not overwhelmingly hothousing the kid) allow kids to play pretty freely their first few years of life. That means that kids are going to do what they are interested in. Some might be desperate to walk/run early and do that while others might want to stack block and work on their fine motor skills. So it shouldn't be surprising if even many normal developing children could be ahead on certain milestones (like walking, for instance).
However, I think it would probably produce better results if they looked at a wider range of milestones and see if there is some sort of correlation between children hitting many of those milestones early (but not all of them, necessarily, considering they might not be as interested in some of them at a young age). It would be quite a bit tricker because parents would have to keep track of a lot more. However, it seems to me you'd have a better chance of predicting giftedness. Just look at these boards on the threads about what your kid did as a toddler/infant. You see a lot of people mentioning various milestones that their kids hit quite early on but those are never going to be the same for each individual kid.
One thing I do find interesting, though, is that the earliest milestones seem to be fairly consistent i.e. that as just newborn the child was very alert, could hold their head up/smile/roll over etc. early. I wonder if that would be more consistent because children at that age simply do not have many options on how they can interact with the world.
