The Savior instituted the sacrament so that believers would partake in remembrance of him. However, these Nephite believers were not remembering. They were physically present with him. Their covenant was with the living Messiah whom they both saw and heard. In future sacraments, they would remember what they saw and heard on this special occasion.
Reference: This verse depends on John 6:35–36, which also provided the more obvious vocabulary of verse 8:
And Jesus said unto them, I am the bread of life: he that cometh to me shall never hunger; and he that believeth on me shall never thirst.
But I said unto you, That ye also have seen me, and believe not.
John announces the bread and water of life and, in the next verse, describes “seeing” and “believing.” While not duplicating John, Joseph’s translation was influenced by that sequence of language and imagery.