Because it would require hardware, network and general security engineers to perform the investigation. And along with that would be a dozen aides or delegates

Because it would require hardware, network and general security engineers to perform the investigation. And along with that would be a dozen aides or delegates to these professionals that would be very familiar with the investigation. It would be obvious if the results were opposite what Cook said publicly, or if the investigation was suddenly stopped and made inconclusive. Apple would also be counting on the US government and Bloomberg to not leak "the truth" so that Tim Cook's statement would not be revealed as a lie. The idea offers intrigue but doesn't pass any kind of reasonable pass of consideration.