Yonkos are seen as on organization, it was less about beating the other Yonko 1v1 and more about overall power. That's why BM believed that allying with Giants could let her surpass WB, those Giants were not going to suddenly help her become stronger in a 1v1.
I cannot understand why people can't differentiate between the two things, there is a difference in how power is shown in the series. There is individual power, 1v1 battle strength, and then there is the organizational power. In terms of organizational power the WBPs were the most powerful, thats why WB was the most powerful Yonko and the strongest pirate. In terms of 1v1 Mihawk is the strongest.