He is not an Uchiha. Uchiha is defined by the clan and bloodline together. When the Sage was alive he was the forefather of the Uchiha, Senju and many other ninja bloodlines, but that does NOT make him an Uchiha. That's like saying because the Romans took over the Greeks all Romans are also Greek. It's a temporally defined signifier for a clan and hereditary traits of the Uchiha, but meant nothing when the Sage was around as far as we know. Your statement is illogical as it backwards. The Sage cannot be considered an Uchiha when no such clan existed until after his elder son had further children and fully developed a distinct Uchiha bloodline.
