Further correction: Brian Mershon has been peddling this stuff for a long time. He makes a great deal out of the fact (which no one disputes or has disputed) that mere attendance at the Masses of the schismatic SSPX does not bring about one's excommunication nor place one in schism. Per JP II, ADHERENCE to SSPX does.
No one incurs excommunication or declaration of schism for merely attending the Masses of the much older Eastern Orthodox schism in its various forms.
Each and every bishop EVER associated with SSPX has been excommunicated and has been in schism. One bishop of the schismatic "diocese" of Campos, Brazil, reconciled with the Roman Catholic Church before his death. This bishop was a successor to Castro de Mayer.
Mershon makes some reference to those who attend SSPX Masses, not from necessity, but for "spiritual comfort." Now where is the line drawn between adherence to the schism and one's eagerness for "spiritual comfort???" If one denies the validity of the Novus Ordo Mass, one rebels against papal authority given by Jesus Christ Himself. A mere preference for Tridentine Masses is common among actual Traditionalist (non-SSPX, non-schismatic, etc) Catholics but, if they are in communion with the Holy See (as SSPX is not), they do not reject papal authority and do not reject the validity of the Novus Ordo.
In more sensible times the SSPX crowd would have been burned at the stake.
No SSPX, No Moto Proprio.