the easiest way to put it is that you embed Lua into your app (say scripts that are loaded at startup or even as a command line) while it’s more natural to embed your external functionality into Python (e.e. numpy).

It’s simpler to connect lua to your c/c++ datastructures than it is to connect Python, and it’s more complicated to write little things in Python.

Sometimes it’s easier to grab a pencil and scribble a little note on a piece of paper. Sometimes it’s a lot more useful to type a note into your phone or computer than to deal with paper.