Again, weak division. And IIRC they’ve also had some schedules that weren’t that brutal. Also, that defense was really great his first couple years. Especially year one, by far his best year.
Yes, he had a solid roster and a great defense, no doubt. That doesn't discredit him as much to me as it does to you. Winning NFL games just isn't easy. Most great coaches in all sports come with great rosters. Very few coaches have carried mediocre talent to the promised land.
On the flip side of the defense, he's had a bad-to-awful offensive line and pretty brutal QB play for most of his time in Chicago. Those are tough things to overcome for anyone. And he certainly deserves some blame, especially for the QB play and playcalling, but not all of it.