| This answer is opinion-based.|
Due to lack of canonical information in the franchise, there is no definite answer to this question.
It's possible this is just an unintentional mistake of the writers of the series where this was mentioned. It could also be one of the changes in the series that simply doesn't follow previous established canon (in this case, from the short Secrets of the Furious Five).
However, though it remains vague to determine, it's also possible that Tigress was only fostered by Shifu as a student and not as a daughter. Their relationship may be more like Shifu and Oogway's; similarly, the series depicted that Oogway found Shifu abandoned in his youth and took him in, became his master, and Shifu was very loyal to Oogway, much like Tigress is to Shifu. Though Tigress may see him as the closest thing she's had to a father, presently neither of them seem to regard each other as more than friends or master and student.