Squeak: The word “B’Shabbos” does not appear in the Yerushalmi (at least, not the one in front of me). Where are you quoting from?
Also, that doesn’t stand up to the fact that there was probably no Eruv, which means they were being M’challel Shabbos. Or that they held like the M’chaber and a disgustingly dirty ball is inherently Muktzah. Neither one applies to ping pong.