A friend of mine is in that diocese, he said he will ask Fr. Prescott why he left the SSPX. I'll let you know when I hear something.
Apparently Bp. Olmstead went to the SSPX priest's retreat this past year and prayed the office with them, so there may be something to this such as a provision where Fr. Prescott asked that he never say the Novus Ordo or something. We don't know the details yet.