There's something to it, e.g. MIT students are overwhelmingly eldest or only children, but I can't see how individual observations like you and I can make would be able to distinguish a 3 to 4 point IQ difference.
Finer measurements and larger sample spaces are required, I'm sure.
I certainly can't tell apart people with a 3 point IQ difference but as the article mentions it is possible that this kind of differences would accumulate over time. For example the difference between an A and a B is pretty small on a case by case basis, but it's a big difference between a straight A student and B average student.
