Defence cap is 539. That means bosses can't land crushing blows on you(crit immunity). Doesn't matter if its Naxxaramas or ICC 25hc. If you go and tank 25hc bosses with your t9 set and 539 defence bosses wont do crushing blows on you, but they will still most likely oneshot you because you simply don't have enough armor/health.
Defence is not a good marker to tell when you are ready for 25hc. It's a thing in early gearing process, that's all.