40K doesn’t really fit. While humanity isn’t the least advanced civilization in that universe, it’s also far from technological dominance (especially considering 10 000 years of stagnation following Horus’ Hoolabooga). I mean, even the vagina faced fish are more advanced than the Imperium!
It’s a combination of two common jokes about them. The vagina-face one is pretty obvious (y’know, the slit), as far as I know, the fish joke comes from the designations for their vehicles (although these are in-universe Imperial designations, kinda like the Hind is a NATO designation for a Soviet aircraft)