The normal style of the Hagadah is that a statement is made and then supported by a quote the Torah introduced by שנאמר. This section should then read, “Our fathers were slaves to Paroh in Egypt and G-d took us out, as stated...”
Further, the entire section is a quote from the speech that Moshe made to the people (Devarim 6) who had experienced first-hand (themselves or their parents) the going out of Egypt. In that context the expression “our fathers who were taken out” makes direct sense.
Rabbi Epstein did not provide an explanation.
ואף אתה אמור לו ההלכות בפסח אין מפטירין אחר הפסח אפיקומון
Why does the answer to the Chacham son specifically refer to this law of the Afikomon as opposed to any other specific law of the Seder?
The last chapter of Pesachim (which brings the entire order of the Seder) concludes with the Mishneh which states this aspect of the Seder, the Afikomon. And this is an advice to the father of the wise son to learn with this son the entire set of laws up to the final detail, as stated in Devarim 6,24, that the father should relate to the son all the Chukim (laws).
And to explain this wording, one should understand that it includes reference to two different time periods: during the time of the Bais HaMikdash and when there is no Bais HaMikdash. Thus, this phrase is understood that when the Bais HaMikdash was standing that after the meal, one should conclude by eating the Korban Pesach; and in the time when there is no Bais HaMikdash, one should conclude with the Afikomon (of matzah) which is in the place of the Korban Pesach. Thus, the shortened version is to be read: Do not finish anything else after the Pesach (the korban in the time of the Bais HaMikdash) (or) after the Afikomon (in the time of no Bais HaMikdash).
