No, he is correct. A tropical climate does not provide the annual cycle of winter dormancy for them to provide quality fruits. They need a warm but dry summer and mild winter and DR just saturates, grapes don't like that.
I don't know enough about it to argue with you about, but again just because The DR is in the tropics doesn't mean the whole country has the same climate. Have you been to Neiba? Obviously the climate is good enough to grow grapes as they they do so and as I understand it is one of the main crops in that area. They are already producing wines and even exporting raw material Uvas de Neyba: Puerta de entrada de Bahoruco al desarrollo so again it doesn't sound that the ability to grow the grapes is the issue.