If they do declare the game a no contest then adding an 8th team to playoffs does nothing. The three teams involved are already in the playoffs so you would still have to go with win percentage to determine seeding. All adding an 8th team will do is give off the perception that the NFL used a player's life threatening injury as an excuse to add extra playoff games to generate revenue.
The idea is that the #1 seed gets a bye, and both the Bills/Bengals had a shot at it. Adding a #8 seed eliminates the bye, so makes some sort of a compromise (we don't know who deserves it, so no one gets it).