Another point to consider is that the SSPV bind the consciences of the laypeople to their non-authoritative conclusions about the validity of the Thuc line bishops.
Regardless of what they think, their act of binding Catholics to their view under pain of denial of Holy Communion is a usurpation of the authority of the Church. They are making a binding judgment, imposing that judgment on Catholics, and by that acting with jurisdiction that they lack.
The SSPV are usurpers. They have no power over any Catholic, and not any authority whatsoever in the Church.