This anecdote gives very little information on its own. Can you describe your experience teaching math to other people - the audience, the investment, the methods, the outcome? Do you have any idea whether that one woman eventually succeeded in learning some of what you couldn't teach her, and if so, how? (ETA: I do agree with the general argument about people who are not good at math. I'm only saying this particular story doesn't tell us much about that particular woman, because we don't know how good you are at teaching, etc.)

Your claim is too large for the evidence you present in support of it. Teaching someone math who is not good at math is hard, but "will in all probability never understand matrix calculus"!? I don't think you're using the Try Harder. Assume teaching is hard (list of weak evidence: it's a three year undergraduate degree; humanity has hardly allowed itself to run any proper experiments in the field, and those that have been run seem usually to be generally ignored by professional practitioners; it's massively subject to the typical mind fallacy and most practitioners don't know that fallacy exists). That you, "in your youth" (without having studied teaching), "once" tutored a woman who you couldn't teach very well… doesn't support any very strong conclusion. It seems very likely to me that Omega could teach matrix calculus to someone with IQ 90 given reasonable time and motivation from the student. One of the things I'm willing to devote significant resources to in the coming years is making education into a proper science. Given the tools of that proper science I humbly submit that you could teach your former student a lot. Track the progress of the Khan Academy for some promising developments in the field.