Sorry to hear of the guy's crash and hope he recovers fully and quickly.
There are SO many variables here that it is nearly impossible to point a finger at the "root" cause. Is it a design flaw with the ST? Doubtful. Most likely it is a series of smaller issues that, when combined just so, cause the tank slapper.
Tire pressure, suspension settings, speed, loading, aerodynamics, chassis design, materials, construction, etc etc all played a factor and unfortunately when they were added up this time the total was bad news. Harmonics are impossible to calculate with all these variables and to just blame Honda is absurd.
Here's hoping that the owner realizes that he was/is the one item in the equation that has the ability to alter the other items and is responsible for the proper care, loading and operation of HIS bike.