Ace's history page says he had defeated Hanafuda after becoming WB's subordinate, but he was invited to join as a Warlord before meeting Whitebeard, so that's impossible as all the spots were occupied.
Hanafuda was a Warlord before the story began Oda revealed in the SBS. He was a smuggler of Ancient Zoans who probably supplied Kaido with a lot of his but the Kaido connection isn't flat out stated. After Hanafuda died Ace was offered a slot but denied it, getting filled by one of the 7 we knew when the story started. I don't think we ever find out who but people smarter than me can probably figure that out
I know that, my point is that on Ace's history page it's stated that he had beaten Hanafuda after joining the WB Pirates, while Draw offered him the position on Sabaody when he met Isuka. Of course, they wouldn't approach a Yonko's subordinate with such an offer.
Why not? If Ace said yes that would've been a crushing blow in the public eye. It would go a long way to showcase the power of the Warlords that pirates would rather be a Warlord than be under the protection of an Emperor. If he said yes the Marines would look unstoppable and hit Whitebeard where it really hurts
