Why would “Eucharist” come to mind if he had never participated in it? The Gospel does say he was a disciple so he would have participated in the “brotherhood” rite. By the way, Otto was a respected theologian and biblical scholar in Germany. His book “The Idea of the Holy” is a classic.
Ah, that is exactly the point. He has no rational basis to recognize Christ in the breaking of the bread, but not face to face. This points to Christ’s presence in the broken bread, i.e. His eucharistic presence.