This was a piece of brilliant misdirection. The Germans, who had intelligence channels suggesting that an operation codenamed Husky was being planned, would now have documentary evidence from a very senior British officer that Husky was itself a cover plan — a deliberate deception being used to confuse the enemy about the real targets. The implication was that the Germans, by focusing on what they thought was the likely target (Sicily), had been successfully deceived by the Allies into looking at the wrong place.
The letter also did something more specific: it referred to a particular named location in Greece — Cape Araxos — which gave German intelligence a concrete geographic target to focus on. The Peloponnese coast near Cape Araxos was an identifiable place with real military significance, and giving the Germans a specific target would focus their defensive attention.
The letter was written in a tone appropriate to a private communication between senior officers — informal enough to suggest genuine confidence, specific enough to suggest real operational knowledge, but not so detailed as to invite detailed cross-examination of its claims. It mentioned Martin by name in a way that integrated him into the correspondence: the letter was being dispatched by a staff officer named Martin who would also be carrying a further communication to the Governor of Malta.
This integration of Martin’s identity into the letter’s narrative was important. It meant that the letter, the carrier of the letter, and Martin’s presence all reinforced each other. Anyone who read the letter would know who Martin was and why he was carrying it. Anyone who encountered Martin would know that the letter he was carrying was of the type described. The letter and the identity were designed to validate each other.