Karin acted like Galu surpassing him 3 years later was a much more recent development than the RRA arc. Unless Karin got some of those MANLY ROSHI GAINZ, he was merely toying with Galu to avoid making getting the water too hard imo. Let's not forget that even though Roshi said Mummy >> 21st Galu, Yamcha lost much more easily to Roshi than he did to the Mummy. I think statements directly comparing the threats of certain opponents are more reliable here than directly comparing stomping feats, since most of the warming up portions aren't good indicators of actual gaps (Yamcha landing a blow on initial Tenshithands etc.).