I doubt that Autocad was ever written in Lisp.
Autolisp was added to Autocad when it already existed. They took the free XLisp (written by David Betz in the early 80s) and integrated it into Autocad. The XLisp of that time was a small Lisp dialect written in C.
Some years ago the old Autolisp implementation was replaced with a different (but compatible) Lisp implementation, which Autodesk bought. That was then called Visual Lisp.
It's confusing, all right, and I don't think it makes for maintainable code.
Interestingly AutoCAD was originally written in lisp and a lot of the drawing objects are represented (or can be accessed) as lisp lists in AutoCAD's own lisp dialect (AutoLISP). It's pretty confusing for anyone not familiar with lisp though and fairly limited compared to other lisps (no macros, as far as I could tell, though I'm sure someone could probably find a way to implement them)