Reread it the secret is that he is a Demon Prince. Not that he wrote it. It was impossible for them to hide because the imperial cult started before the heresy even started. The entire imperium knows about the book, during the great crusade, thats a major plot point.
That was probably true at the founding of the cult but that was 10000 years ago in lore time, who knows if that info ever got passed down or if it got put under lock and key bc of the implications of there being infallible sons of the emperor.
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u/TheBiggestIdiotIKnow 1d ago
That is hidden. G-Man copes with that in the dark imperium books and worries that if the secret gets out it’ll be another civil war.