I also thought they didn't do flex yet. Hmm i found some info
In 2006 the NFL implemented a primetime "flexible scheduling" element on Sundays in Weeks 10-15 and in Week 17. In 2014 that was extended so that games between Weeks 5 and 17 could be affected by flex scheduling. However, only two games may be flexed into Sunday Night between Week 5 and 10.
ONLY TWO GAMES can be flexed between weeks 5-10 so yeah no chance they pick a Seahawks home game you know better than that.