> It's not the mathematical concepts they're struggling with in this example, it's the notation.

Being a mathematician, programmer and psychologist, I can hardly believe that you can rip math notation apart from math itself. To be fluent with math one needs three "languages" which her mind can use in parallel to think about math problem. Its math notation, English (or any other natural language) and visual mental image. If any of such a mental languages is not available to a person, or if he is unable to translate from one to other, then he would struggle with math.