He did when he was still working on indie French Canadian stuff. He actually talked about this before. He doesn't think he's a great writer, but he knows what he wants out a story. English isn't his first language either. So he prefers to work closely with writers and focus on the directing.
Honestly this isn't a bad approach. Not every director
needs to also be the writer of their script. Hell, Ridley Scott is the same way. Both Alien and the original Blade Runner were written by other people.
From what I hear about Denis, he's apparently really big into the concept of collaborating. He loves to get input from anyone that's around on set and see if anyone has an idea of how to further improve the film they're all making together.
Which, in my humble opinion, is part of being a director.
It's all about walking that fine line between being humble and also having a strong vision for what you're setting out to make your film to be. Not everyone is going to always have a better idea than you, so you need to be ready and able to hold your ground on it if the situation comes up... buuuut you also need to be able to let go of your own ego a bit and consider that maybe someone else has an idea of how to improve upon what you're making.