TLDR: the Ultramarines were caught with their pants down because they can't quite grasp the concept of not 'fighting by the book'. The Word Bearers by comparison held no such scrupples

How on earth is that the conclusion you draw from that excerpt? Are we reading different stories or something? It's nothing to do with being caught with their pants down or not fighting by the book. The entire problem with the Calth war is that they literally can't be everywhere at once, they're understrength. They don't have adequate numbers to both guard their caves and also mount attacks on other places.

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Incredibly weird take OP, your bias is showing hard.