In looking at the opponents for the Seahawks and Niners, the two teams only faced two different opponents. The rest were common opponents.
The Niners faced the Eagles and Packers. The Eagles finished four games over .500, as did the Packers. Both finished with 10-6 records.
Meanwhile, the Seahawks faced the Bears and the Giants. The Bears finished 11-5, six games over .500 and the Giants finished 9-7, two games over .500.
Unless I am missing something, both the Niners and Seahawks played equal strength, since their differing opponents finished with a 20-12 record.