The sophistication of this system is one reason why the Mincemeat documents worked as well as they did. German intelligence was not receiving a single anomalous piece of evidence pointing to Greece and Sardinia. It was receiving multiple apparently independent confirmations of the same picture. The documents were coherent with what double agents were reporting, with what diplomatic sources were suggesting, and with what simulated military preparations were implying.
This also means that attributing the shift in German defensive dispositions specifically to the Mincemeat documents, as opposed to the broader Barclay deception, is difficult. The documents were the most famous and well-documented component of the deception. They were the component that Montagu could describe in his postwar account. But they functioned within a system, and the system’s overall effect was larger than any single component.
The Ultra intelligence that allowed British planners to monitor German assessments and adjust the deception in response to enemy reactions was the feedback mechanism that made the system function. Without Ultra, the British would have been operating blind, unsure whether their deception efforts were having any effect. With Ultra, they could see German assessments changing and could calibrate their efforts accordingly.
The decision to bring Mincemeat under the operational umbrella of the London Controlling Section, which coordinated all British strategic deception, reflected this systemic approach. The LCS was responsible for ensuring that all deception efforts were coherent and mutually reinforcing. A deception operation that contradicted what double agents were reporting, or that implied a timeline inconsistent with what simulated military preparations suggested, would be detected and would undermine not just itself but the entire system.