Yes, that's exactly how BB does things but I bet Teach wouldn't expect Luffy, not even in his wildest dreams, to pull a win against two Yonko. BB has no idea how strong Luffy is, he doesn't know Luffy has the samurai and the minks are on his side, he doesn't know about Kid, and he doesn't even know BM's crew is taken out of the equation. So, for BB, Luffy's crew+Law's crew went on a suicide mission to fight against Kaido's and Big Mom's crews, which means there is no way BB would plan an attack on Wano. He is far more likely to attack WCI just like you said.