Stir limoncello and simple syrup in a small bowl. Unroll cake, remove towel, and place on parchment on the back of a rimmed baking sheet. Brush with limoncello syrup. Spread with semifreddo, leaving a 2-inch border on 1 short side and a 1/2-inch border on the long sides. Freeze until semifreddo is firm but not hard, about 25 minutes.
Starting at the short side with semifreddo spread to the edge, roll cake, using parchment to help roll it. Transfer rolled cake, seam side down, to a platter; freeze until semifreddo is hard, at least 3 hours or up to overnight. Let stand at room temperature about 30 minutes before serving. Serve dusted with sugar.
