While DC cleaned up any mess from poor defense by last year's guards, I think you're discounting the impact of Steph's defense compared to McNeeley. Steph could be counted on to shut down the best guard/wing on opposing teams and is looking like he'll be doing the same in the NBA for a long time.
When your best offensive player gets stopped repeatedly they become a non-entity and it frustrates the whole team. McNeeley hasn't shown (yet) that he'll be doing anything close to that. In his defense (pun intended), he's still adjusting to the speed and athleticism of the college game but Steph was a great defender from day one, with or without DC in the game.