Was not the continued de-synchronising of engines to confound acoustic detection a bit of spin to obscure the discussion of radar detection?
The synchronising of an aircraft's propeller speed is practical and desirable: comfort, vibration, human and equipment fatigue etc but synchronising the props of one 'plane with those of another is not possible. Thus a group of aircraft within the range of a detector would sound the same to that detector whether or not each aircraft had synchronised engines.
Some interesting pictures and some notes about acoustic aircraft detection here
Acoustic Radar.