I already said it on a diffrent thread. But let me repeat it again here:
Fighting a stronger guy doesn’t necessarily mean that the one is stronger than the other. For example: Killer fought Hawkins, which is a headliner, while Robin fought BM, which is from the flying six. From the feats we really can assume that BM was stronger than Hawkins. But you should be stupid if you assume that Robin is stronger than Killer just because she fought a stronger opponent. The same can be said about Nami, who fought Ulti.
So if we say Law and Kidd are stronger than Zoro and Sanji just because they fought a emperor, while the other two fought YC is not strong evidence. Because if that is the case we can also say Nami and Robin are stronger Killer.
A second thing I want to add is:
The way Big Mom is defeated does play a role about the powerscaling of both captains. If we only look at the result and not to the method than yes “Law and Kidd are both stronger than Zoro and Sanji”. Because defeating a emperor means more than defeating a YC. But! From the same logic we can assume that all of the BB pirates are toptier, low yonko tier because they killed Whitebeard. It doesn’t matter whether WB was old, tired or hurt. The result is a fact. BB pirates killed WB!
Nobody in this forum will acknowledge this statement and argument. So if this is not possible for the BB pirates, how can we say that Law and Kidd have actually beaten Big Mom?!
For last and not the least:
If both Zoro and Sanji were to attack with there Ultimate move I actually do think that they would have beaten Big Mom for real. Not just hurt her.
Zoro; who cut the wings of the strongest skin known till date and Sanji; kicking one the most tanky cyborg Zoan (literally) out the Island do have the attack power to beat up Big Mom. So if we were to switch the players. The result would have been different.