The following is a slightly revised version of the treatment of the Papal Bull, Cum ex Apostolatus Officio, taken from the soon-to-be-released 2nd edition of True or False Pope?
Canon Gregory Hesse, who held a Ph.D in theology and canon law teaches the same in this video.
“...they boldly affirm that the Roman Pontiff and all the bishops, the priests and the people conjoined with him in the unity of faith and communion fell into heresy .... Therefore they deny also the indefectibility of the Church and blasphemously declare that it has perished throughout the world and that its visible Head and the bishops have erred. ... These unhappy men undermine the foundations of religion, overturn all its marks and properties, and invent so many foul errors, or rather, draw forth from the ancient store of heretics and gather them together and publish them. Yet they do not blush to call themselves Catholics, and "Old Catholics"… these men having progressed more boldly in the ways of wickedness and destruction, as happens to heretical sects from God’s just judgment, have wished to create a hierarchy also for themselves, as we have intimated. They have chosen and set up a pseudo-bishop, a certain notorious apostate from the Catholic faith, Joseph Hubert Reinkens.
"So that nothing be lacking in their impudence, for his consecration they have had refuge to those very Jansenists of Utrecht, whom they themselves, before they separated from the Church, considered as heretics and schismatics, as do all other Catholics." (Pius IX, Esti Multa).
As Pius IX noted, after splitting from the Church following the First Vatican Council, the Old Catholics turned to the Jansenists of Utrecht - who they acknowledged as heretics before leaving the Church - to have their first bishops consecrated.
Similarly, after splitting from the Church following the Second Vatican Council, the first Sedevacantist sect (the CMRI) turned to the Old Catholics - who were declared heretics by Pius IX, and who they themselves acknowledged as heretics before leaving the Church - to have their first bishop (Francis Schuckardt aka Antipope Hadrian VII) consecrated.
The similarities between the Old Catholics and the Sedevacantists are striking, as we have seen in previous chapters. In yet another similarity, they both appeal to the same problematic and now defunct papal bull, Cum ex Apostolatus officio, to "prove" the central point they use to justify their separation from the Church.
* Canonist Fr. Coronata also explains that Cum ex Apostaolatus is null: Institutiones Iuris Canonici.