What those unfamiliar with the Passover feast are ignorant of, is that Jesus used the 3rd cup, and the hidden half of the center matzah (the Afikomen) to perform the first Communion. He REVEALED HIMSELF in these two pieces symbolically, revealing their purpose.
But their presence within the Passover has been from time immemorial - right back to it's institution in Egypt.
The third cup is The Redemption Cup... The Fourth, The Cup of Thanksgiving is taken later... There is more to come. The Passover feast must still have relevance, as it has not yet been perfectly revealed.
Christ instituted nothing new, but only revealed the Communion within the Passover - He is the promised "Redemption". It is part and parcel of Gods appointed Holy Days.
I am still learning, so I may not be speaking with perfect accuracy, but without investigating the Passover as instituted by God (and all the other appointed Holy Days), one cannot see the truth.
I encourage you to research the Passover, and wonder why Jehovah would have left it undone - substituting a different ceremony in the context of the Eucharist or Communion.
The answer, ultimately, must be that He wouldn't. His Holy days are "shadows of things to come". They are part and parcel, part of the Prophecy. Those who reject them do so at their peril.
Ergo, the entire scheme of the Eucharist/Communion are anti-Christ... "replacement" for the real thing. To say otherwise, one must first explain the fulfillment of every single action and symbol of the Passover, and that cannot be done.
I’m not really up on this, you are right. But I believe that Hahn maintains it was the 4th cup that Jesus gave as His blood. I hope someone more knowledgeable can come to hold up his side of the discussion.