[mauriciolange]

This was a code key, not a codified message, so there was nothing to break, but only the indication that messages could have been exchanged using this key.

[adastra22]

Which is gold material for the code breakers. The people trying to decrypt messages having to do with the ongoing war at the time.

[quietbritishjim]

Yes but if the codebreakers also came across a (real!) coded message, wouldn't they want to have the key already to hand so that it can be decrypted?