You can tell something of a persons training by watching the roundhouse and the hook kicks.
Those who train primarily for the sporting aspects of TKD will kick with the instep and the ball of the foot, respectively.
Those who train primarily for self defense will kick with the ball of the foot and heel, respectively.
I've seen references from the KKW showing both methods.
In the videos with the KKW-released iPhone app, the kick is demonstrated as being done with the ball of the foot.
In
Complete Taekwondo Poomsae by GM LEE, Kyu Hyung and GM KIM, Sang H, the roundhouse is described as being with the ball of the foot.
The video you posted, which is a clip from a DVD series released by the KKW, also shows it as being the ball of the foot.
However, in this video featuring GM LEE, Kyu Hyung (current head of the KKW and long time director of their poomsae team), you can see that the roundhouse is done with the instep. I would say this is a clear indication that either is acceptable, at least for poomsae.
At our school, we teach both techniques with both kicks. Students are taught that the ball (or heel) of the foot must be used against hard targets, with the instep (or ball) of the foot being used for poomsae or striking soft targets. In sparring, lower ranks use the instep (or ball) of the foot because it lessens the chance of injury.
We require that breaks done with the roundhouse be done with the ball of the foot. With the hook, they must be done with the heel. I've done breaks with the instep, but never more than 2".
Ask your Master what he wants to see in your school for testing and competition purposes and do it that way.