I believe the parasympathetic nervous system (PNS) has minimal or no effect on the ventricles of the heart. I recall in Dubin's Rapid Interpretation of EKGs, he mentioned this when he was talking about sinus rhythms, junctional rhythms, and ventricular rhythms, and asking the readers "How could we go from sinus rhythms -> ventricular rhythms?" and he talked about how a lot of drugs stimulates the PNS or the vagus (X) nerve being stimulated, but the PNS doesn't affect the ventricles of the heart.
Then I believe Atropine antagonizes acetylcholine (ACh) blocking the receptors for it.
Perhaps he was thinking that over stimulation of the PNS was an issue, but since it's a trachycardic arrhythmia instead of a bradycardic arrhythmia, in my mind, the issue wouldn't be too much PNS stimulation, which I think we'd expect to see a bradycardic arrhythmia (ventricular escape focus), but rather the ventricular cardiocytes are excited from an issue like hypoxia.