Actually, if you look through the comments at Yglesias’ post, Steve Sailer, who I normally have somewhere between very little and no use for at all, makes some good points about why it is more likely that there is a selection effect for intelligence which is stronger for O-linemen than for wideouts or D-linemen. Namely (and I am clearly stealing his logic here, so all due props), if you look at the population of tall, fast men with good hands, high jumping ability, and good quickness, you are looking at a very, very small population. So if you want a wideout, you have a small pool to choose from already. Add in a requirement for high intelligence, and you might find very few, indeed, who meet the bill.
Similarly, for a D-lineman you want someone who is tall, strong, and very quick, since speed is of the essence when you are looking to sack a QB, while speed is less important in an O-lineman. The population of people who are “tall, strong, and very quick” is, by definition, smaller than the population who is “tall and strong”. So, since there are more potential O-linemen to pick from, you can get choosier about other attributes, such as intelligence.
It’s not a proven case, but the argument certainly makes sense to me!