Apart from this being very elementary instructions, I think the bigger story is that the CIA is abusing its power to classify information so extensively that simple writing advice was classified for 55 years.
That case sounds like a textbook example of a document where you want to use that trick.
Such practices have a major effect on people regardless of how aware they are of what's actually going on.
- United States Marine Corps, School of Advanced Warfighting
So yes there is a very good reason to keep the CIA writing advice classified. Even though it seems banal.