You are correct in that surveys of IQ and other intelligence scores consistently show physicists having some of the highest. But mathematics, statistics, computer science, and engineering are the same, and most studies I've seen generally see very little, if any, significant difference in intelligence scores between these fields.

'Rationalist' isn't a field or specialization, it's defined more along the lines of refining and improving rational thinking. Based on the lesswrong survey, fields like mathematics and computer science are heavily represented here. There are actually more physicists (4.3%) than philosophers (2.4%). If this is inconsistent with your perception of the community, update your prior.

From all of this it is safe to assume that the average LW'er is 'very smart', and that LW contains a mini-community of rationalist scientists. One data point: Me. I have a PhD in engineering and I'm a practising scientist. Maybe I should have phrased my initial comment as: "It might be better if the intersection of rationalists and scientists were larger."