According to the Catholic magisterium, are sedevacantists by definition schismatics? This is both a terminology question and a Catholic doctrine question regarding the status of sedevacantists, that is, those who reject the current pope as illegitimate.  I found the following Catholic definition of "schism" in Canon §751:


  schism is the refusal of submission to the Supreme Pontiff or of communion with the members of the Church subject to him.


This seems like it could plausibly applied to sedevacantists by the "official" Catholic Church, but I'm not sure that it actually is.  The penalty for schism is severe – excommunication, per canon §1364 – and I haven't heard of this being applied.

So my question is, according to the post-Vatican II magisterium of the Catholic Church – that is, the official teachings of the pope (Francis, Benedict XVI, etc.) and his bishops – are believers of sedevacantism necessarily in schism with the Church?  If not, why not?  And if only some are in schism, where is the dividing line?

Note that I'm focusing here on the magisterium – not the views of sedevacantists themselves, nor those of theologians who recognize the legitimacy of post-1970 popes.  Here I'm interested in the official teaching of the Catholic Church (as led by Francis, Benedict XVI, John Paul II, etc.).
Bishop Ngô Đình Thục, a sedevacantist, was declared excommunicated, but not considered schismatic. (He did not die in the state of excommunication, though.)

On how excommunication does not necessarily imply schism, cf. p. 60 of An Open Letter to Bishop Clarence Kelly on the "Thuc Bishops" and the Errors in The Sacred and the Profane by Mario Derksen, M.A..