A lot of people on HN just don't really know any stupid people. There are many adults in the US who struggle to work a grocery store self-checkout machine or remember a string of numbers longer than 5 or 6 digits. (I'm 70% sure the correct order of magnitude for this group is "millions" and 99% sure it's at least "hundreds of thousands").
Maybe with better nutrition, childhood conditions, and healthcare a good portion of this group could have been promoted into a different group, but the idea that everyone just needs better prereqs and they'd learn math better isn't right. The article itself was written by someone who manages their own website.
It is possible to teach math to this population, I've done it briefly when I was a teacher in an adult education school.
It's significantly slower than for average people and requires a ridiculous amount of effort on the part of the teacher and student, but it's possible.
Due to limited working memory many of these students will make mistake no matter how difficult the material is, and they will be slow at resolving problems, but I've never had a student without serious mental health issues be unable to get to precalculus level, and in the ones I've seen try they've been able to get to university math.
Again, many will always make more mistakes and/or be slow, but the concepts themselves are generally within grasp.
Maybe with better nutrition, childhood conditions, and healthcare a good portion of this group could have been promoted into a different group, but the idea that everyone just needs better prereqs and they'd learn math better isn't right. The article itself was written by someone who manages their own website.