Shabbat addition

Hello Rabbi,
Last Friday (the seventh of a holiday) I arrived at a synagogue in the afternoon where a Mashiach meal (of Chabad) was being held. One of the Chabadniks tried to convince me to eat matzah, and after some discussion I remembered that I had accepted a Shabbat tosef on the way to the synagogue. With a feeling of gratitude to God, I informed the Chabadnik that I could not eat since I had entered the Sabbath and was obligated to make kiddush. He, for his part, claimed that a Shabbat tosef is not from the Torah and that for the sake of a mitzvah (he includes the Mashiach meal as such) I am exempt from the Shabbat prohibitions of this tosef. I later saw that a Shabbat tosef is from the Torah and that in tractate Rosh Hashanah it is even required from the Torah (I believe by inference), but even this argument of mine was met with his response that although the mitzvah itself is from the Torah, the prohibitions that apply to me therein are not from the Torah. His first reason was to point out the attitude of the poskim towards the additional Sabbath time and the various permissions they offer when necessary, and his second reason was that the Sabbath prohibitions are applied from the Torah in their own time and objectively, and I cannot choose when to apply them.
I would love to know the Rabbi’s opinion on the matter.