I tried this by saving the file on my computer (I used Chrome but as far as I understand the browser is irrelevant) and importing it in TeXmacs. I got an IMO acceptable TeXmacs document.
Most inline math in the Wiki text is written using italic text, so it is not possible to recognize that automatically.
The display math could be improved if one would be able to transform all “description” environments to display math environments (at the same time removing the italic tags).
Maybe someone is able to do that using TeXmacs?
For the other page (StackExchange) one gets a slightly worse imported document, which could be improved if one would be able to remove automatically all labels that are generated converting MathJax expressions. Here too I must leave the word to someone else, as I would be able to do it using different programs, but one would want to do it in TeXmacs.
Perhaps a Scheme program, but let us hear someone else.