I think they used MPI (the m4-like one) because it is much easier to retrofit as a sort of templating language for use in web pages back in the 90s when that was a thing, as opposed to the modern practice of maintaining an entire parallel-universe DOM which is then transformed into the real DOM. MUF (the Forth-like one) was a bit too low-level for the application, though I do seem to recall a story this same furry wrote in which a fantasy CPU is mentioned that runs MUF natively, like a Lisp machine but for furry roleplay environments. I won't get into the uses to which it was put.