That's not what I'm thinking of either. I sort of remember an interview with her at the time her episode aired in which she maybe, possibly said that she couldn't do Arrow earlier (not 415, prior to that and to her episode) because she was working on a movie. I could have fabricated it from 10 different stories, though.
ETA: It seems I have mixed things up. According to this interview, she was asked to direct for the first ttime this season.