I haven't had time to do any amateur research on the references in the book so I don't know. But the explanation does make since on the heart benefits of K2. And to be fair I believe the way the author worded it was K2 was the only vitamin proven to reduce plaque and not that it was the only way or only substance to reduce plaque.
My cardiologist likes vitamin D, K2 and L-arginine for cardiovascular health. (Proper K2 supplement dosage appears difficult to determine.)