Intelligence is largely similar between all humans: we don't have actual boundaries.
You do know what a bell curve is? Sure, most people tend towards a mean but the difference between either end is immense, with very real implications. It separates hedge fund managers from janitors. Different races and ethnicities also tend towards different means. Half a century of trying to eliminate "the gap" between blacks and whites (about a standard deviation in IQ) has been a dismal failure. Billions of dollars has been thrown at this money pit with nothing to show for it. We will see commercial fusion reactors, strong AI, heck, even mass-market-popular commercial flying cars before the gap has been eliminated.
IQ tests follow a sliding scale such that Einstein was kind of dumb and we've repeatedly revamped the Culture Fair and changed the baselines for the Wechsler.
I am definitely on the right side of the bell curve, I was born a lot later than Einstein, and modern physics is still one of the hardest subjects I've taken, if not the hardest. I call BS on this one. If Einstein did not so great on an IQ test, it says more about the particular IQ test than Einstein's IQ. I suspect that there were questions on the IQ test where Einstein was right and the IQ test was wrong, and/or the IQ test was only calibrated to be accurate near the mean and not where Einstein's IQ was. You can take a hundred cram school attendees who have managed to ace the SAT through sheer bloody-mindedness and still not get the intellectual output of one Einstein.
Attaching sounds, smells, and actions makes a more vivid, accessible, memorable image; and complex techniques and systems such as linking, story forming, and mind palaces further aid in recall by providing indexing or association.
I know the technique of mind palaces and find them utterly unwieldy. Why use a mind palace to remember a fact when you can just write it down or google it?