And also, I don't think Schiff is a very good interpreter of Beethoven. Have you listened to his Moonlight sonata lecture and his reasoning for why he interprets the music as cut-time? Yikes! It sounds pretty bad played that fast.
But, most people would interpret adagio sostenuto as slow and sustained. There is nothing sustained or slow in his interpretation. Also, it's possible that it was printed incorrectly. The original manuscript is missing the first page so there is no way to check what Beethoven really wrote as the time signature.But back to fp and Schiff's interpretation. He has a tendency to think that the score is gospel, even if it contradicts musical intent.