The statement of Rav Yosef seems to answer why "Im Hayah Choletz Choletz l'Pesulah" but not why "v'Im Hayah Meyabem Meyabem l'Kesheirah". Why can't the Yavam do Yibum with the Pesulah?
If, as we currently understand, Pesulah refers to Pesulah le'Alma, then what the Tana means is that whereas the Yavam is obligated to perform Chalitzah with the Pesulah (like Rav Yosef explains), he is permitted to perform Yibum with the Kesheirah, should he so wish.
See Tosfos Yom-Tov & Tiferes Yisrael Perek 4, Mishnah 11.
Be'Virchas Kol Tuv,