With Kaido's statement about Haki and Roger, I have a theory. Kaido first admired Xebec being strong and for having DF. He thought having a great DF would make you strong and that's why he beg BM for a legendary fruit. Then years after that, Roger became a Pirate King by just having great Haki without DF. By that time Kaido realized that he just need better Haki and so he honed his Haki. Also, this might also prove that Roger was actually stronger than Xebec.