I think this means that if the merchant does not have a written return policy MasterCard assumes the
de facto policy is "no returns". This seems sensible because merchants generally have no obligation to accept returns, and it is the customer's responsibility to check before buying. However, the situation changes if the product is defective, and the transaction could then be formally disputed if it meets the criteria in the
Fair Credit Billing Act, and this requires adhering to a very specific written procedure.
Telephone calls will not preserve your rights.
A salient point! OP has not mentioned he has the dispute in writing, yet. AFAIK, the dispute must be received within 60 days from the statement on which the charge showed up in order to preserve your right.
Still, we would like to know the issue of the quality that makes the OP unhappy to return a $2K product. I am very skeptical that Visa would side with the cardholder as Chase said, without going thru full investigation as MC now seems doing. Chase CSRs might just offer an easy answer to wash their hands off this dispute.