I can accept this with regards to the ritual sacrifice of animals, since that ritual has passed away completely with the one incomparable Sacrifice of Our Lord, but attending a seder hosted by a Jew is a little different. There are seders that wouldn’t be appropriate, because the Haggadah used is in violation of Christianity: that is, if the ceremony says plainly that the Messiah has not yet come. So many Haggadahs have been written since, though, including by Jews* intending* to include Christians, that a Jewish seder that does not violate Christianity is possible, by leaving questions open upon which Christians and Jews do not agree.
Is that watering down Judaism? That is Judaism’s issue. We have to realize that Judaism does not have a well-defined deposit of faith such as we have. So while some Jews wouldn’t sit for a seder that doesn’t clear disavow Christianity and the possibility that Jesus is the hoped-for Messiah, while his point probably applies, then, to a seder that would be acceptable to most Orthodox Jews, that is not all seders.
Early Christians kept celebrating seder, separately from the Breaking of the Bread, for many years after the Mass was instituted. Just as it is not wrong for a Christian to be circumcised, it is not wrong, per se, for a Christian to attend a seder. It is only wrong under particular circumstances.
I think the way the speaker differentiates Biblical Judaism from modern Judaism in order to make his point is a stretch. The Temple that was destroyed was the *Second *Temple. Jews had been in diaspora and were without the Temple Sacrifice for a time before the coming of Christ and the building of the Second Temple. That didn’t mean that Judaism ceased when the Temple Sacrifice was taken from them, either the first time or the second. I think his understanding of modern Judaism is at cross-purposes with what the Magesterium teaches about modern Judaism, as well. He almost treats Jews as pagans. That creates more confusion than it cures, IMHO.
It is well for a Christian to read the Haggadah to be used before deciding to participate, though. It is desirable to thank God for the Exodus, and it is fine to join Jews in doing so. The line is crossed when any disavowal of Jesus as the one Savior sent by God is part of the seder. In that case, it is necessary to decline to participate.
I’d repeat, too, that this is a Jewish ritual. A group made up of Catholics alone, having the Mass, wouldn’t do it. Without a Jew to lead it, the meal would have no raison d’etre, for the common ground for which we praise God would have a far higher common denominator.