From what I can tell it's the opposite (on Firefox): the math is in the source HTML twice, once in <span class="katex-mathml">, which is hidden using CSS in the non-reader mode but is rendered in the reader mode, and <span class="katex-html" aria-hidden="true">, which is what FF displays normally, and it's missing from the source when I inspect it in reader mode. Having two spans like this comes directly from KaTeX, whose authors I'm sure have thought about accessibility. I imagined that MathML would be somewhat standard and screen readers would understand it.
Huh, you're right about what the Firefox reader mode does. I'm somewhat surprised it uses the MathML. I'd just assumed what was happening from the 2pi in the header becoming plain text but that header is probably a special case and just from the page title rather than the h1.
