I think that assuming that the design is correct It means that I don't have to consider the possibility of having a wrong design, so I don't have to consider Design Failure mode but just Preocess ones. AmI wrong?
Think of it this way: If the design is "correct," the task is to try and understand potential design/product failure modes with that assumption in mind.
BUT, in the course of the DFMEA process, it might be discovered that there is something questionable about the design, despite the assumption of correctness. That discovery should be reported to the proper design authority.