What third factor producing both higher IQ and then industrialization are you suggesting?

Obviously you're not suggesting anything as silly as the industrialization causes all observed IQ changes, because that simply doesn't explain all examples, like East Asian countries:

A crucial question is whether IQ differences across countries are a simple case of reverse causation: Do high-income countries simply develop higher IQ’s? We address this question in a number of ways, but the most important is likely to be this simple fact: East Asian countries had high average IQ’s—at or above the European and U.S. averages—well before they entered the ranks of the high income countries. This is precisely the opposite of what one would expect if the IQ-productivity relationship were merely epiphenomenal.