seudas Purim on Shabbos - different reading of the Yerushalmi

Before leaving Purim behind and looking toward Pesach, I have to add one last point to an issue I raised in a previous post. In discussing whether we pasken like the Yerushalmi’s chiddush that seudas Purim cannot take place on Shabbos, my assumption was that “al hanissim” aside, there is not much of a big deal in eating a seudah both on Shabbos and on Sunday to be yotzei all opinions. I take that back! R’ Friedman (Rosh Mesivta of Mesivta Rambam in Lawrence) quoted a chiddush of R’ Soloveitchik that the Yerushalmi does not just mean that there is a chisaron in the kiyum of seudas Purim if done on Shabbos, but the Yerushalmi means there is an issur from the perspective of hilchos Shabbos in holding a Purim seudah on Shabbos day. Very interesting lomdus. I am just wondering though, assuming the Yerushalmi means there is an issur, how exactly is the issur violated? In what way would the seudah be defined as a seudas Purim as opposed to a normal Shabbos meal (seems strange to say kavanah alone is the issue here)?