You're really saying they were an average defense over the year? I agree this is an ugly game for them today, but over the season they were one of the top defenses in the league...
numbers-wise, which is how things like defense is rated, they were a top team. But I agree that when you look at actual performance, which actually matters, they've given up too many leads in the 4th quarter this year.
Another case of how numbers don't matter when the results are terrible.
(why Bradford is a loser)
But the Seattle D isn't bad; they just aren't as good as they have been in earlier years. A lot of that is due to the start this year, though, when Kam was out and because of injuries.