But wasnt there some supreme kais stronger in combat than other ones? I´d say as far as supreme kais are concerned, there´s a great diversity in their magic arts and warrior traits, though, the ones that got rekt by Buu were mostly warrior types, there were magicians like Babidi who could pull some interesting magics even with shitty power levels (even his barrier, could allow him to survive a SS2 majin Vegeta suicide bomb
), so it´s more like, depending on what magics Piccolo can learn he can become more of a threat, like lets say he trains on making warping space-time spells on Whis level, that would make him without a doubt a threat, but if he´s stuck with the average sealings and telekinesis alone, he would have a hell of a time trying to survive Buu level, oponents.
Now though, there was something that Piccolo didnt undergo in his training as well, if I recall, he never went through any time of limiter removal boost, that Guru and Elder kai performed so he could get a additional power-up there, which should always be anything below mystic gohan, but a somewhat significant power-up nonetheless