It has been a while since I learned that gemara, but I remember learning pshat that Rashi is referring to a non-Jew that makes a formal kabbalah in Bais Din to become a ger toshav. Part of that formal kabbalah is that he will keep Shabbos, but it is not one of the 7 mitzvos for an ordinary non-Jew who has not made such a kabbalah.
(note this doesn’t really answer Tosafos’ question from ??? ???? ???? ????)