And the version of the slur you're talking about is a pretty old one that I have not heard used in decades (of course that may be regional, generational, etc.)
But like you, I have never heard "mongo" used in the same pejorative context.
It is a very common slur among kids in Norway and originally means someone who has the "mongoloid" syndrome. These days it just means "you idiot" after "pro choice" succeeded where Hitler "failed" and got rid of most people with downs syndrome.
Oh wow, I had no idea! I've never heard of this. Thanks for letting me know.
I only knew it as the name of "Planet Mongo", the main planet in the Flash Gordon universe, an old science fiction comic that has been rebooted many times (which also had loads of extremely racist anti-Chinese elements)
In the Scandinavian countries "mongo" is used as both a slur and a general insult. However, since it manages the feat of being both ableist and racist at the same time, its popularity has waned quite a bit over the years.
Though I do love the dictionary definition of "mongo":
"Items found in the trash that can be salvaged."
Sums up my views of mongodb pretty well, actually.