May I disagree? 2 cups sifted = 2 cups sifted flour or 2 cups of sifted flour , however,
2 cups of flour sifted = sift after.
In this case: "2 cups sifted all-purpose flour", the flour is sifted first, then measured because the measurement is for 'sifted flour' rather than '2 cups of flour, sifted'.