Probably destiny modification. The spell changes the past so that Octavia always intended to perform here today, and the mariachis somewhere else. The most powerful magics in the show (dark or alicorn) all seem to work that way.
I did not expect to see the mariachi band return! Of course, given what’s just happend, now I wonder if we’ll ever see them again. Will poor Mrs. Blue Mexican Pony and the kids haveta learn to accept Octavia as their new spouse and parent?