I mean you can put several people on one pedestal without them being absolutely equal. The group Kaido put on one pedestal as the ones capable of fighting him are Xebec, Whitebeard, Roger, Shanks and Oden. And I doubt Oden is exactly = Roger, Whitebeard and Xebec and even Shanks. So it would make sense for Garp to put Rayleigh in the same breath as Whitebeard, as they are close power wise, which doesn't mean that either Whitebeard or Roger wouldn't edge Rayleigh out. Likewise, Big Mom was said to be Kaido's equal, when ultimately, Kaido is stronger.