There seems to be a lot of confusion out there about what a sedevacanist is and how the definition applies to SSPX
Before I let SSPX answer that, here are a few talking points on sedevacanists.
Sedevacanists reject the Magisterium of the Catholic Church. They have their own Magisterium.
They reject the authority of Christ's Church and the authority of the Pope.
Sedevacantism is the theological position of those traditional Catholics who most certainly believe in the papacy, papal infallibility and the primacy of the Roman Pontiff, and yet do not recognize John Paul II as a legitimate successor of Peter in the primacy. In other words, they do not recognize John Paul II as a true pope. The word Sedevacantism is a compound of two Latin words which together mean "the Chair is vacant."1Sedevacantism appears then to be a theological position or a theory kept by some traditional Catholics who think that the most recent popes, the popes of the Vatican II council, lost their pontifical authority on account of the grave heresies they have been promoting, and the crisis that came along...
Survey of the Theological opinions of a heretical pope: For that survey, I will follow the study of Arnaldo Xavier de Silveira in his book ‘La Nouvelle Messe de Paul VI: Qu'en penser’ (or: LNM)3. After explaining how the New Mass departs from the traditional teaching of the Church, this author makes an in-depth study of the theological hypothesis of a heretical pope. Such study was highly praised by Archbishop Lefebvre, as "the very objective study of Xavier de Silveira". .
Just in case you remain unconvinced that SSPX are sedevacanists, here's more:
It consists of saying that a heretic cannot be head of the Church, but John-Paul II is a heretic, therefore he cannot be a true Pope.
Here they are calling Ratzinger and future popes heretics.
This is the exact substance of their defection,
SSPX can claim they are against sedevacanism all they want. It is intellectual dishonesty. It's what they are doing and therefore what they are.
The lifting of their excommunication was for the purpose of trying to bring them into communion - which in my opinion is an exercise of futility because they reject the authority of the Church and Pope.
In my opinion they are worse than sedevacanists because they are telling Catholics what they're doing isn't sedevacanism.