The Terminus decree really exposed the lore secondaries for not knowing much about the lore and just want to complain.
A codex isn't objective information, it is a primer on the army written from in-universe sources. Novels are (unless shown not to be such as the PoV of an unreliable narration) objective information and already told us what the Terminus Decree is.
Also take two seconds to ask yourself, how does the author of the codex know what is written in the box? Only the Grandmaster of the Grey Knights can access it, and he is currently stuck in the warp 99% of the time.
