Certainly expanding the cohort alone is sufficient to cause a drop in average IQ scores - which is exactly what I said. College is no longer a useful proxy for IQ score. I made no claim that college would cause IQ scores to increase and if they do play a role I don't think it is a very large one. Raw scores are fitted to a normal distribution but the IQ scores are normalized across time by comparing that distribution to other distributions. Certainly some processes for obtaining IQ scores can be more flawed than others.
There is a reverse Flynn effect (peaked for children born in 1975) so not only are average IQ scores lower for college grads but there has been a drop in IQ for the general population.