SpaceX concluded that the weight savings by having the grid fins permanently extended and the attendant simplification of the actuator mechanism, increased performance even considering the added drag. With the diameter of the vehicle being 9 metres, the frontal area of the grid fins is small compared to the rest of the ship. Also, during ascent, drag is significant only during the first portion where the atmosphere is densest. On descent, the vehicle is moving faster when it encounters the dense lower atmosphere, so the grid fins have the necessary control authority.
In the first of the four part “Tour of SpaceX Starbase with Elon Musk”, Elon Musk said future versions of Super Heavy may reduce the number of grid fins to three or maybe even two, as that should be sufficient for guidance during the descent phase. This harks back to the Canadian bush pilot aphorism, “What you don’t have can’t break.”