Yes, the M language is used for the rules and calculations, though you have many other applications in python. This presentation[0] gives more info about why. TL;DR it started in the '80s.
Maybe it's more like Github doesn't pay proper attention to Google? Just guessing though, I haven't actually checked Github's SEO, and wouldn't event know where to begin.
The StackOverflow guys on the other hand have always said how they designed the site especially for Google, and keep on optimizing it accordingly.
[0] https://forum.openfisca.fr/t/presentation-du-code-source-en-...