Medley Interlisp is a living fossil more than the mataphor implies.
The build process doesn't compile the Lisp sources but mostly combines existing compiled files generated up to a few decades ago. Almost a third of the Medley sources haven't been compiled since Jan 25, 1998, and many of the sources dated from years before. It's as if your current Linux distro was built from .o files compiled 27 years ago.
How's that for maturity and stability?
More on the build process:
https://github.com/orgs/Interlisp/discussions/2316#discussioncomment-14678446