Rocks didn't have Sky splitting moment before this chapter. It doesn't mean he can't do it before this chapter. It's just a sky splitting moment, not Roger/WB moment or whatever. Roger vs WB was more impactful and devastating than this one. Xebeck wasn't at his strongest when he clashed with Harald, and Kaido put Oden alongside Roger, WB, Shanks and Prime Rocks. Not that I think he's comparable to them, but he's not less than Harald was when he clashed with Rocks.