It's a textbook example of why the Yonko's don't fight in the first place, because no matter who wins, the other one, and their crew will be too injured to withstand an assault from another Yonko/the Marines.
Aramaki isn't stupid. Even if he knows Luffy is weaker than Kaido, he knows that he is still strong enough to go toe to toe with him, and powerful enough to be able to put him down under the right circumstances. He also knows that Luffy is still recovering from this massive fight. This is the best time for him to strike, because once Luffy and his crew recover, they will be much, MUCH harder to deal with.
He's not here because he thinks the man who just put down Kaido will be a cakewalk to fight. He's here because he knows that this is the best chance they have to put down the man who just took down Kaido, because that man and his crew is still injured.