Earlier quoted context omitted.
MDL is what was in wide use at MIT at the time, the PDP-10 era. The M in MDL is sometimes "MIT" in the various backronyms of what it stood for. (Mostly it was apparently just short for "muddle", a self-deprecating description.) (Also, to be technically correct, these source files aren't even MDL, they are a further descendant called ZIL [Zork Implementation Language].)
Yea, I get that MDL came from MIT, but I have to think that MACLISP was far more used at MIT at that time. But maybe not.
(Also, in reading other comments around here, I've learned there's a deeper connection in MDL to Scheme than I knew before, so I hadn't realized the Lisp/Scheme split even has ties to this "competition" of Lisp languages at MIT.)