Can you cite your source because I have ratings that show the Bill-Steelers games was the most-watched individual game that weekend with a 10.47 share, 18.1 MM viewers. Perhaps you're referring to the Sunday 4pm ratings of 13.12 rating/23 MM viewers, but that's for ALL games shown, not just the Cowboys.
http://www.showbuzzdaily.com/articles/skedball-weekly-sports-tv-ratings-12-9-12-15-2019.html
Further, Bills-Dallas drew a 13.53 rating, 32.6 MM viewers. Top rated football game.
http://www.showbuzzdaily.com/articles/skedball-weekly-sports-tv-ratings-11-25-12-1-2019.html