Yes, they didn't want her to dance, because she was injured. But they still wanted her to perform on stage while sitting down. I see that many people had done that before (like Gikwang, Minho & Hyosung had appeared once for a performance after healing for a bit), but I guess on the last minute, Hwayoung didn't want to do that on Music Bank since there was only two performances left. And therefore, Eunjung and Hyomin had to learn her rap instantly. That's why Hyomin kind of struggled on it since it was soo last minute.
However, I just really want to know the truth too, especially from T-ara.