Which is to say, the original authors didn't care and were just writing latinesque gibberish that sounded cool.

One of the critically important things to remember about 40k lore is that the game was originally a joke. Their "real" game was Warhammer (the game to be later called "Warhammer Fantasy"). Originally it was just "Let's put space opera tropes onto a game board and have them fight D&D monsters". Then they shoe-horned the chaos stuff in, then had a (presumably drunken) exercise in lore writing where they "invented" "Grimdark" by essentially turning the Despair knob up to 11.

And then it turned out to be popular, and it was time to start trying to write real stories on top of the mess. And... people loved it. Including a lot of people for whom the comically ridiculous Grimdark stuff turned into a kind of authoritarian wish fulfillment, which wasn't great. But you have to remember it was all a joke to begin with.