Someone please tell me this is transpiled from a separate project.
If I remember correctly it was in fact written in Common Lisp; the output was originally that file but it may have been modified since. You can probably google the truth with those breadcrumbs :)
It's originally written in LISP and this is why it's a single C++ file. However, I believe that it's now being maintained in its C++ form.
Python's main interpreter loop is a single 4k line function.