maybe they just made a mini-lisp and called it MDL?

It’s very Lispy, but it’s not strictly Lisp. Why, for instance, use “<“ and “>” to surround various forms but not others? If they were to make a mini-Lisp, I’d expect something more like Gnu Emacs Lisp, something that’s obviously a Lisp, but heavily influenced by the Lisps of the day. I’ve found a few old MDL manuals linked from Wikipedia, but none of them have any sort of “Here’s why we created MDL” section that I could find.