As it is impossible to change someone's Imperial linage to an immigrant, someone might have forged the story, considering that the loyal subject Takanori KOJIMA being a naturalized person was not good.
もともと皇胤の血筋である系図を渡来系に変えることはまずありえないから、結局忠臣児島高徳が帰化系とあっては具合が悪いと感じた人々が、伝承を作り変えたものであろう。