His defense hasn't improved at all. If anything its gotten worse in recent years(Last year was the worst of his career). Losing Landy as his winger has really impacted that part of his game. I dont think people fully understand how much heavy lifting Landy does on the defensive side of the game(Going to be such a huge boost when he comes back).
All that being said, who cares when he's on pace for 130 points and ultimately who cares about the regular season. Playoffs are where it matters and while he's still not great defensively, the effort level on that side of the puck ramps up to a 10 from him when the games start to matter, and that makes a huge difference with him.