I am not sure why you would be given this advice, as the Courtauld is known for producing many scholars and leaders in the art world. I suggest checking the bios of current students at schools where you want to apply for a PhD. I believe there are Courtauld MA grads at Yale, Columbia, Berkeley, IFA, and many other top programs right now. That is not to say that Tufts wouldn't prepare you for graduate study, but it does suggest that a combination of academic skills developed at the Courtauld and their name recognition has helped place students at these programs. Of course, the Courtauld's special option system means that you have to be more sure about your area of focus than in a traditional MA program.