First: That was marketing.

Second: The line was actually “The Saga is Complete.” Which was true, regardless of viewing order. Before ROTS was released, only episodes 1, 2, 4, 5, and 6 existed, so the Saga was incomplete. With the release of ROTS, all six episodes were available, so the Saga was complete.

Third: The tagline for TPM was “The Saga Begins.” If “The Saga is Complete” signals that ROTS was intended to be watched last then “The Saga Begins” must also signal that TPM was intended to be watched first. How are you going to make that consistent? I don’t think you can.