I can't say this as an absolute, but around the time of leaving the FSSP, I was looking to be conditionally Confirmed and had contacted the SSPX- they were not addressing conditional Confirmations any longer ( or at least not at that time) I was unable to get further information. I was getting Bishop Williamson's newsletter and read that he would be in the States shortly for Confirmations. I made arrangements to be there and from then on have been aligned with the "Resistance". I was conditionally Confirmed 45 years after my Novus Ordo Confirmation.
Might it be that the SSPX does not want to intimate that there might be anything wrong or lacking with the Novus Ordo Sacrament(s)?