I originally booked our reservation for the Boardwalk Villa Studios for two adults through the Chase Disney Visa Card Room Only Discount in October 2018. I called again in December 2018 to add a third adult and was charged an additional per room night fee for the third adult.
Your book advised that DVC properties did not charge a fee for additional adults beyond two in a room.
When I called to question that, I received conflicting answers. I requested to speak to a supervisor who reiterated that a 3rd adult resulted in an additional fee per night, and that my source was indeed unofficial. However he did say that he would look for another discount to lower my room rate, and came back with a rate that was cheaper than my original two persona rate. So please check your sources regarding DVC room rates (he said one or two bedroom villas did not charge an additional fee for adults, but studios do).
It worked out okay for us in the end, but please check on this for future editions.