No. All we know is that the electric potential is constant within this region of space.
Work Step by Step
Equation 23.18 calculates the potential difference between 2 points by using the electric field along a path that joins the 2 points. If the electric field is zero everywhere in the region, the potential difference is zero for any 2 points within the region, i.e., we know that the electric potential is constant within this region where E=0. However, we cannot conclude that the constant potential equals zero here; the reference level where the electric potential equals zero is arbitrary.