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.
When they all were taken mainly by domainers they were going crazy that they'd be the new LLL.com's where the prices went from $xxx to $xx,xxx in a very short period of time.
However, what has happened with LLLL.com's is that they went very quickly to $xxx but have since dropped considerably so you can acquire LLLL.com's (4 letter.com's) from $15 upwards although if you want a really nice LLLL.com such as Made, Ball etc then those prices are different regardless of it being an LLLL or not.
Just have a look on Domainer forums such as Namepros, DNF, DigitalPoint etc and you can find some really cheap.
And you can get them for cheap. Got one .com as recently as last year, with the .org as a freebie. I'm now considering selling my 3 letters/numbers domain name - .net and .org.
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-...