> I’ve taught mathematics at community colleges for over 20 years and I’m absolutely convinced that not all people can learn algebra or calculus.

This is the "As a mom"-argument applied to iq. When it comes to child-rearing mothers will not infrequently claim to know what works best in general because they have had children of their own. I mean an equally valid interpretation of your anecdata is that not all people can learn how to teach mathematics at community colleges.