I'm not too deep into 40k lore, but I think it ended up being something along the line of classic anti-communist brainworms, about how any good ideology is actually just a smokescreen so a small elite can rule over everyone with lies and manipulation, because obviously a society couldn't actually pursue a noble goal without there being an ulterior motive. Also, collectivism bad, individualism good, Tau bad for disrespecting :freeze-peach: and forcing ideology on others.