This isn't a Nocturnals song. Grace was hired to play a character on the show so this song, I'm assuming, is a song the "character" might sing/write. There is absolutely no reason to think it would/should be a Nocturnals song.

Perhaps I'm wrong, perhaps the character Grace was hired to play is "Grace Potter". If this is the case the "Grace Potter" she is playing still needs to fit in with the theme and writing of the show.

As to the Noc's not playing on the recording of the song. Given that Grace flew down to the set to make it perhaps it was just cheaper to hire other folks to play it rather than flying the band down? Who knows . . .