Yes this a winter closure issue. Notice also the warning: 'This route includes roads that are closed in winter.'
It won't let me put a destination point anywhere on the West Entrance - Norris - Mammoth road (except the junctions). So that's flagged as closed. Looks like someone entered incomplete, and possibly unnecessary, road closure information for the park.
Ideally it should let you specify when you are traveling, but for now it's taking the 'safe' choice and assuming you want to travel now. Programs like this have gotten a lot a flack for recommending impossible or unsafe routes.