Interesting theory, but based on how he reacted to the formation of the Grand Fleet, I doubt he’d then make Yamato do some shit like WB did to Oden just to prove what she says is true, he’s not really like that. Bartolomeo, Cavendish, and the others never asked to specifically join the Straw Hat Pirates, they just pledged their support as allies both before and after Luffy told them how he felt about the idea; I don’t recall any times when someone explicitly asked to join his crew and he turned them down, so it’s hard to believe the big brains around here who think he’s suddenly gonna do something super out of character to someone who couldn’t be more clear about wanting to travel with him