What makes you think their Defense was bad?
Last year, before the season started, there were many touting it as one of the top defenses. Then a couple of guys in particular (Hanifin and Faulk most notably) really struggled defensively. The team as a whole didn't give up many shots, but Bill Peters played a system that limited shots, but was susceptible to odd man rushes. He had the defensemen playing very tight gaps in the neutral zone and pinching aggressively in the O-zone. While this helped with possession, it also meant when a mistake was made, it ended up in a good scoring chance. The goalies weren't great (particularly Darling), but IMO, they gave up too many high scoring chances. I can see why people would question how good the defense was based on that. It's fair. Even with that, Slavin and to a lesser degree Pesce, were very good.
That was a different D than this year though. This year, they replaced Hanifin with Hamilton, added de Hann to the top 4, and Faulk (so far) looks much better than last year. Throughout pre-season (doesn't mean much) and the first couple of games, they looked pretty good defensively, but the last game was a free for all pond hockey so if someone just watched that game, they'd easily draw a conclusion that may or may not be valid due to small sample size.
They are very young and have a lot of new players and a new coach, so I suspect we'll see some inconsistency from them early on. They have 3 guys at center, which is important to team defense, that are just learning the ropes of playing center in the NHL (Aho, Necas, Wallmark) so that will play into it as well.