Society of St. Pius X Is Not Sedevacantist

No they are not, because sedevacantists believe there is no valid pope. The Society of St. Pius X (SSPX) recognizes the validity of all legitimate popes, including Pope John II, under whom its founder, Archbishop Marcel Lefebvre, and the four bishops he unlawfully ordained were excommunicated in 1988. In 2009, Pope Benedict XVI lifted those excommunications, though Archbishop Lefebvre had died in the interim. The SSPX has not yet been fully reconciled to the Catholic Church, including because of some doctrinal issues. For more on the SSPX, click here and here.