Ellenville is further south, down the Rondout Valley. There would likely be additional traffic from the region east of the cliffs traveling west from New Paltz along Rt. 299. But, the bulk of the traffic from the north would come down RT. 209 from Kingston, and from the south from Rt. 17 through Wurtsboro, or from Rt.52 from Newburgh via Pine Bush and Cragsmore.
This concern will not dissuade the powers making the final decision. The bigger concern will be for those coming north to the Gunks via the Thruway past the Harriman Toll Plazas, having to deal with the vastly increased traffic coming north from the metro area. That will be a complete cluster fook far worse than your typical Friday trip north, or your Sunday evening drive home. And, that will happen regardless where the final two casinos are finally built.