Although Fu's body was identical to Raiden Ei's, there were certain internal differences. For instance, the Raiden Shogun's body had been modified to be more compatible with the Musou Isshin, whereas Fu's body was Raiden Ei's original, unmodified form, which had a much lower compatibility with the blade.
As for "Transcendence: Baleful Omen", although it dealt Electro damage in the game, in reality, its execution relied on the deployment of the Plane of Euthymia to combine spatial slashing with Electro damage.
Since Fu did not possess the Plane of Euthymia, she naturally needed to develop a new skill. After all, this wasn't a game where one was restricted to a fixed set of abilities.
Chapter 182