There's been some confusion here, but his comment was: "The chapters running right now [391-400] and the next 30 were all written four years ago, so I want to show what we are drawing n—"
Obviously 401-410 have been storyboarded and (mostly) drafted, but it's not clear how much has been done on 411-430. I imagine some of them are storyboarded (especially in the 411-420 batch), but we can't know for sure.
I'd be surprised if the arc wrapped up with 430, but 440-450 definitely seems possible (if he's able to get that far).