Question:
Answer:
The issue is one of canon law. The Eastern Orthodox Churches are completely separate churches. Similar to how Eastern Catholic Churches are separate from the Latin Church, but in this case in communion with each other.
The clergy of the SSPX are clergy of the Latin Church. As such, they are bound by the canon law of Latin Church. Clergy of the Eastern Orthodox Churches are not under the canon law of the Latin Church.
Canon law requires that priests have what’s called “faculties” in order to validly hear confessions (can. 966 §1). Priests usually are granted the faculties through their local ordinary. Because clergy of the SSPX were not in full communion with the pope, they essentially had the status of a suspended priest and thus did not have the faculty to validly hear confessions. However, as you noted, the Holy Father has recently granted this to them.