From this we see that priests must have permission of the Church to absolve sins. The Church, by the way, gets to determine how the sacraments are administered. The SSPX does not get to decide how sacraments are administered.
Also, we have to consider culpable and inculpable ignorance. Catholics ought to inform themselves about their Faith. To what extent is a matter for debate. But once you walk through the door of exploring your Faith even to the point of learning about the law and faculties that priests have, I think you are on the hook. You don’t have to wonder ever about priests at the local parish or official chapel established by the local diocese. Even Father “Just call me ‘Bob’” has faculties, even though he is a heretic.
That said, if a person has been informed that SSPX priests do not have faculties to receive sacramental confessions, and goes to them anyway, a huge problem is introduced. Those priests don’t – in normal circumstances – have faculties. Period. Some people say they have “emergency powers”. The Church does not agree. They don’t have faculties.
It seems to me that if a person knows that the priest does NOT have faculties, and he goes to him anyway, then he knows that he is simulating a sacrament. That would be a sin.