Pope St. John Paul II defined SSPX as a schismatic movement and summarily excommunicated its ringleaders (LeFebvre, Castro de Meyer, de Mallerais, Fellay, Williamson and de Gallareta). That Benedict XVI saw fit as a matter of ecclesiastical mercy to lift the excommunications of those still living and not the dead Lefebvre and did NOT lift the declaration of schism and did not affect SSPX's status as schismatic. Subsequently Fellay and the the other survivors, upon what "authority" we may only speculate, purported to excommunicate Williamson, the Holocaust denier.
Note that the notorious heretic Maguire feels no compunction to oppose SSPX, SSPV, or the "churches" descended from the Utrecht heresy of the mid-19th century. Nor do we often hear SSPX, SSPV, etc., objecting to heretics like Maguire. What have they all in common? Contempt for papal authority. Natch!