I have been to San Sebastian 5x and this Airbnb is hands down the best place I have ever stayed in. The wraparound terrace is just out of this world, you wake up every morning to the view of the concha beach on one side, and the mountainside and city on the other. There is nothing like this in the world.
You are also right across the street from Old Town so you can access the most delicious food in less than 3 minutes. Don't overthink it, and just book this amazing apartment.