Ball milling exposes surfaces to fresh reactive radicals introduced through tribo-electric effects.
Ultrasonics just vibrates and could be combined with ball milling to improve mixing rates, but without physical friction, it probably won’t be as effected.
In the process described they're using metal balls - so there won't be any electric effects. The metal balls immediately conduct electrons to negate any charge imbalance.
You're thinking of non-metal balls, used for other processes, e.g.
Ultrasonics just vibrates and could be combined with ball milling to improve mixing rates, but without physical friction, it probably won’t be as effected.