But why did Jiang Xinghuo say "an emperor who is as heroic as his father and great-grandfather"?

The only conclusion that can be derived from this sentence is that his eldest son Zhu Gaochi will inherit the throne in the future, and Zhu Qizhen's father is Zhu Zhanji.

Otherwise, if the second son Zhu Gaoxu inherits the throne, he should at least be "an emperor who is as heroic as his grandfather and great-grandfather", and Zhu Gaoxu should not be skipped.