I don't think their assertion is that someone with 40+ years of math experience would have difficulty learning new notation, it's that LaTeX is composed of a lot more than straightfoward notation. LaTeX can get pretty complicated pretty quickly. While it's basic math notation is quite fantastic, it's the basic document formatting that I think would give most people trouble when learning.

Because the error messages from latex are truly awful, even just putting an underscore or less than in plain text is painful.

Assuming they have something that already works for them, lack of motivation.

Because knowledge about maths is independent of computer and programming literacy.

if we use a liberal definition of "computer and programming literacy", then it's just as much a requisite for math as it is TeX.

if we use a conservative definition of "computer and programming literacy", then it's not a requisite for either.

markup languages are not programming languages. most people have some familiarity with some markup language.