Michael Matt is the editor of the anti-sedevacantist newspaper The Remnant. Below is how his endorsement of Salza/Siscoe’s book reads. 
“For 50 years we at The Remnant have fought against the false conclusions of the Sedevacantist thesis. We have insisted that, despite the revolution, the Church is still ours–our castle, our home, our mother and we cannot abandon her. Taking their lead from St. Athanasius during the Arian crisis, John Salza and Robert Siscoe have elevated the discourse of this pivotal debate to an entirely new level that encourages Catholics to keep the old Faith and fight for our Church under siege. This book serves notice to the occupiers of the Catholic Church: Traditional Catholics are not going anywhere. We’ll stay and we’ll fight until all of “our buildings” are in the hands of Catholics once again.”
Editor, The Remnant newspaper
Mr. Matt begins by saying the conclusion of sedevacantism is false and ends by endorsing the conclusion of sedevacantism.
The conclusion of the sedevacantist thesis is that Francis and his supporters have the Church under siege and have taken over “our buildings.”
Salza/Siscoe’s book is saying Francis I and his supporters are Catholic.
During the Arian heresy, the Arians were not members of the Catholic Church. It’s the sedevacantists that follow St. Athanasius’ lead, who taught to shun the Arians and to avoid those who worship with Arians even though they themselves hold to the true Faith. (Patrologia Graeca, Vol. 26, pp. 1185D-1188C) 
Michael Matt is one of those that worship with the new Arians today as he condemns the position of St. Athanasius that sedevacantists are following.
True or False Pope Refuting Sedevacantism and other Modern Errors
 Latitudinarian Maxims – The Divine Law on Catholic Communication in Religion with Non-Catholics, p. 49, Steven Speray, Confiteor.