That's not really the whole story. He was still hard to balance even after they removed the cooldown reset, and he was strong even in situations where the smaller ult reset didn't come into play. The bigger problem was that his Q was just completely guaranteed to hit, and all of the damage was done non-conditionally (yeah, you had to hit with the hilt, but that was easy with the instacast). If you got behind at all, you just couldn't get near Darius without taking a large guaranteed chunk of your health. They made it dodgable and took damage off of the initial hit, then gave him the potential to get more damage over a long fight. They also gave him another sustained fighting mechanic in the heal.
They'd be gimmicks by themself, but together they create an actual playstyle. It's working much better than Old Darius ever did. Much easier to balance.