An interesting read, thank you.
It's not hugely different, Treacher being an old fashioned version of Traitor, (as in Treacherous).. but it has a pleasing archaic quality that sits really well with the Heresy (or Inter-Legionary War) being a tale from myth.
I imagine it fell out of use because of the endless arguments about pronouncing it "tretcher" or "treecher".
Interesting wee detail there that the chaos marines aren't immortal/time-lost corrupted veterans of the Long War, but their weird, mad and mutated descendants...