Also, if this thing is truly mangling bytecode, it's not portable between
    different python versions [...] seems to be done by interpy_untokenize,
    which boils down to some string mangling

It uses the Python file encoding property ("# coding: foobar") to rewrite the source code, not the bytecode, and they refer to pyxl as an inspiration.

For a good explanation, see https://github.com/dropbox/pyxl