Cackle (Su): A witch can cackle madly as a move action. Any creature that is within 30 feet that is under the effects of an agony hex, charm hex, evil eye hex, fortune hex, misfortune hex, or ward hex caused by the witch has the duration of that hex extended by 1 round.
This seems rather open ended, and I'm sure the intent is as you say. Though I can't imagine a ruling any other way, usually restrictions like that are openly stated.
Though mentioning warding seems off, did Warding used to have a duration shorter than when hit or fail a save?