You cited already from the relevant sentence, but you need to read the whole sentence:

Unless the binding region is canceled, all threads of the team that executes that binding region must enter the barrier region and complete execution of all explicit tasks bound to that binding region before any of the threads continue execution beyond the barrier.

The last clause of the sentence provides the synchronization for implicit tasks. All threads and therefore all implicit tasks in the team must enter the barrier before anyone can leave the barrier. "Complete execution" would not make sense for implicit tasks, because the implicit tasks will continue execution after the barrier.