The arrogance is astounding. The vast majority of people you're talking about have absolutely no interest in studying in those fields. Those that try and fail, were unable to create/learn the inexplicit knowledge required to understand everything. That does NOT mean they cannot, in principle, create/learn that requisite knowledge, merely that they failed to do so. When someone makes an error, we never assume they are doomed to forever make that error. We can correct our errors. There is absolutely no difference between a simple error correction, and an extremely large complex error correction, except for scale. If someone can understand explanations for one thing, there is nothing, in principle, stopping them from understanding anything else. You're essentially advocating for supernatural thinking. That there is some special magical thing about complex explanations that means only certain people with special intelligence can understand. That is just not true. We are universal intelligences, and given enough time, anyone can understand anything. I readily admit that some people are quicker and more efficient at understanding, whether it be because of the inexplicit knowledge they create as young children, or because their memory and processing power is higher. But taking much longer to understand something is very very different from being in principle unable to ever understand something. Unless someone is severely disabled, they are universal in their ability to understand.

His point is pretty simple to understand: there is no qualitative leap between the brains and minds of people who do well at MIT and the common healthy bloke. He isn't claiming that anyone could "do well" in those schools (because it would imply performing at the same level on a battery of standardized tests... which basically are a proxy for IQ testing. Since no one here is claiming that we all have the same IQ, your rebuke to his position would qualify as a strawman.

Regarding "good memory", it actually is pretty much the gist of it. It's not about having good long-term memory (the ability to "memorize" stuff) but sufficient working memory performance (the neocortex's distributed "RAM"); which has been observed to strongly correlate with IQ. To make it short, the main differences amongst humans that are relevant to the IQ distribution seem to be quantitative in nature (mostly, working memory performance, which itself is highly dependent on white matter integrity i.e. myelination of neuronal axons).

Notice that I didn't say "working memory size" because, as the research shows, these resources are scattered over such a sizeable portion of the brain that the relatively tiny differences in unit recruitment wouldn't explain much of the experimental data within the prevailing theories. So yeah, I'm talking about short-term memory encoding/decoding performance, here.

I know it's a hard pill to swallow but if you want to rely on "intelligence" to explain that phenomenon, then you'll lose your biggest opportunity to argue for qualitative factors as the main drivers of academic performance. In fact, working memory performance (which is much more straightforwardly quantitative than intelligence) is an even better predictor of academic success, especially at higher IQs (interestingly enough, the scenario that would be more relevant to this AGI/ASI debate).

Finally, since we're playing this game, I also went to an engineering school (studied AI), so don't expect your appeal to authority to work here. Let's be real about STEM classes: that shit might be "HARD" but it ain't witchcraft. It's also ironic that you used the driest and most clear-cut subjects as examples. It doesn't strengthen your point.