Or is the idea that IQ stops strongly predicting achievement above 120 wrong?

Most IQ tests are not very well calibrated above 120ish, because the number of people in the reference sample that scored much higher is rather low. It's also the case that achievement is a function of several different factors, which will probably become the limiting factor for most people at IQs higher than 120. That said, it does seem that in physics, first-tier physicists score better on cognitive tests than second-tier physicists, which suggests that additional IQ is still useful for achievement in the most cognitively demanding fields. It seems likely that augmented humans who do several times better than current humans on cognitive tests will also be able to achieve several times as much in cognitively demanding fields.