The validity begins from the moment that the censure is lifted. It is not retroactive.
It’s not the sacrament that is invalid. It that’s the priest involved cannot validly celebrate them because canon law says that a priest must receive faculties either from a Bishop of a diocese or a Major Religious Superior. Bishop Fellay is no longer a Major Religious Superior within the Catholic Church, because he is excommunicated. Therefore, he is no longer an ordinary and cannot grant faculties as the superior of the Jesuits or Dominicans can.
These young priess are stuck in limbo. They are validly ordained priests, but with no one to grant them faculties to hear confessions or witness a marriage and on top of that a prohibition from the Church that forbids that they celebrate any sacrament, even the ones that they can celebrate validly.
The Superior os the SSPX seem to have committed a great act of injustice to these young priests.
Maybe this is what makes the bishop the the Society of St. John Vianney a more credible person. He wanted his priests to have the full benefits of being a priest without ever doubting their status within the Church. He did not want to prolong the separation from the Church.
Nor did he attempt to criticize the Pope and everyrhing the Pope taught. He simply wanted the EF and the opportunity to train priests in the EF.
By submitting with great humility toward the Vatican and great charity toward his priests, he got what he wanted and a little more.
JR