I haven't read this book, but I can't see how Schelling would convincingly make this argument:

Leo Szilard has even pointed to the paradox that one might wish to confer immunity on foreign spies rather than subject them to prosecution, since they may be the only means by which the enemy can obtain persuasive evidence of the important truth that we are making no preparations for embarking on a surprise attack.

It's true that enemy spies can provide a useful function, in allowing you to credibly signal self-serving information. However, deliberate, publicly-known policies of aiding enemy spies defeats the purpose, because at that point, it's indistinguishable from counterespionage. After all, why not go one step further and feed spies truthful information? Same problem applies here.