I didn’t. I took it out from Rashi. Rashi (Vesuccah Tihye) says that S’chach is only that which is meant for shade. The next Rashi says that the issue is that we don’t need the S’chach. On Amud Beis (Tarti) he writes that the term Succah is only used when it is built for shade.
Rebbi Zeira is saying that since in such a structure the shade is the walls and not the S’chach, the S’chach in this case is not put there for shade.