The difference is Yamato never looked better than Kaido. Garp looked better than Kuzan with every on panel exchange while being significantly weaker than his prime self. Again, Oda went out of his way to personally tell the readers that this isn’t prime Garp here, that guy’s a different beast. He couldn’t have worded it even clearer than that. Garp said he was also at half strength at one point in the preskip, an exaggeration most likely but it goes to show that their prime selves are significantly more powerful, bare minimum. He treats the old legends on a higher pedestal, he might as well draw them standing on a stool every time they make an appearance.