"To fit the guitar into the story, I had to change the script a little bit. Jack came by car and had left the guitar in the car, so after we start recording again, Jack will say he's going to get the guitar from the car and we'll cut to the scene of Jack singing the song to Steve. Angelina began to explain. "Since Noah is a singer who came as a guest artist, putting a scene of him singing in the episode could be good for keeping his audience for the show, as well as adding to Jack's character in the show."
With that explanation from Angelina, most people accepted it.
Singers' audiences were much more fanatical about seeing new and different work than actors' audiences, because actors' audiences were used to seeing them perform all the time, so many didn't bother to see everything that artist did.