I do believe this.Do I believe there is no cut content in season passes? No. Sorry, but no.
However most likely some guys are already working on DLC before base game release. You need to consider the development pipeline. The base game is nearly finished, so you have some guys that have worked in earlier development stages that don't have tasks right now. If you want to keep and pay them you need to give them tasks. That's why DLC makes sense not only for additional revenue but to keep a steady workload until the next game.