Son of - you are totally correct - the exception only applies to duty periods of 12 hrs or less.
Unfortunately CAP 371 because of the way it is written is open to mis interpretation by both sides - it really wants re vamping in a more readable and logical format.
Computerised systems with the rules set up by "experts" with non overridable "hard" legal rules are the only answer to get everyone tuned in to the same frequency.