Unity is an essential note of the Church and is essentially focused on the only visible Head of the Church, the Roman Pontiff, to whom rests the principle of apostolic succession (or formal Apostolicity, while the only 'material apostolicity' is not enough as a note of the Church of Christ). Then, without Peter or Pope there is no Church, which is in communion with Christ through the Prince of the Apostles. [6]
Therefore, everything that happens outside the uninterrupted chain of Peter and his successors is outside the Unity and formal Apostolicity of the Church [7] and reveals the detachment of dried branches from the vital trunk of the Church of Christ.
But the entire Traditionalist movement rests upon a foundation that has been precisely NOT in communion with the Pope to begin with (after Vatican II Council); so unless this priest happens to be a diocesan one in good standing and not affiliated in any way with the SSPX, Resistance, etc., this argument against Sedeprivationism seems null.
There is really nothing new here but the same old "Recognize and Resist" disputations, which the author clearly endorses.