Even before I noticed this feature, I expected the IQs to be inflated because, well, they're self-reported

Yes, and even without particular expectation of inflation, once you see IQs that are very high, you can be quite sure IQs tend to be inflated simply because of the prior being the bell curve.

Any time I see "undiscriminating scepticism" mentioned, it's a plea to simply ignore necessarily low priors when evidence is too weak to change conclusions. Of course, it's not true "undiscriminating scepticism". If LW undergone psychologist-administered IQ testing and that were the results, and then there was a lot of scepticism, you could claim that there's some excessive scepticism. But as it is, rational processing of probabilities is not going to discriminate that much based on self reported data.