They are undecided about what they believe. If they knew what they believed, they'd be either theist or atheist.So, undecided people do believe in God. Either one holds a belief in God or they don't. Being undecided is still not holding a belief in God.
To be clear, I thought you referred to them as atheists when you described them as, "they do not believe in both the existence and nonexistence of God." And it appears you are. However, I don't find anything rational in the idea of denying "both the existence and nonexistence" of an object.