John 8:16

Yea, and if I judge, my judgment is true; for I am not alone, but I and the Father that sent me.

This proves that John 8:15 was no disclaimer of his right to judge. His oneness with God was proof that any judgment by himself was not merely his, but God's judgment also.

For I am not alone ... This was addressed to the slander that Jesus' witness of himself was to be rejected. His witness, on the other hand, was plural, both his and the Father's witness concurring.