The sacrament is eaten in remembrance of the Atoning Messiah. They were not remembering. They were right there 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 in this special occasion.
Reference: It would appear that the final structure of this verse depends upon the New Testament context that provided the more obvious vocabulary of the previous verse:
John 6:35-36
35 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.
36 But I said unto you, That ye also have seen me, and believe not.
The Johnannine context has the statement of the eternal bread and water of life, and the very next verse has an emphasis on “seeing” and “believing.” While not a duplication of the Johnannine text, this verse appears to have been influenced by that sequence of language and imagery in John.