The sushi bar is not open during lunch hours. I can't remember if they open at 5pm or 5:30pm, but that's roughly the time they open the sushi bar.
I believe the only places you can get sushi for lunch would be either the restaurants in the Japan pavilion at Epcot or Wolfgang Puck Cafe at Downtown Disney West Side. If I'm forgetting any other locations, I'm sure someone here will be sure to correct me.