If it was that obvious, Oda would not make such a mystery about it. He maybe attacked Big Mam's ship because it was on the way of something he desired and judging by how fearless Kid is, he could have attacked it just for the sake of it. Not necessarly mean it's his target. Not saying it's impossible to be the case, but you never know
Read what Law said:
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To survive the new world you either go under the umbrella of a Yonkou or continuously challenge them.
Kidd was doing just that against big Mam, and now he formed an alliance to do so.
Law and Luffy's alliance is to take out kaidou "supposedly," which luffy boastfully said he will take them all out(obviously not gonna happen).
X Drake came into the new world immediately seeking Kaidou's attention, later on he started working for him.
No Supernova has ever challenged Shanks specifically, not even during Ace's time that was heard of YET.