Is One Piece a manga about pirates fighting in a race to find the treasure that is the series' namesake? Yes.
Is finding the One Piece the end of the series? Yes.
Is Buggy (and by extension Cross Guild) more important than the Marines, Revolutionaries and WG forces when it comes to finding the One Piece? Yes.
What's so hard to understand about this? Buggy isn't gonna be stronger than any of them, or more impressive, or more talented.
But by simple virtue of the series' endgame being what Oda has said countless time, he's gonna be more important in the long run. Simple as.
We don't have to overly focus on one panel like that out of context, nor do we have to assume the *entire* series is about finding the treasure, despite how important it is. There's other stuff going on like:
Imu's plan to flood the world(maybe this is linked to the OP,but we don't know)
The question of justice, how it can be defined, who will define it etc(Sakazuki, Dragon, Kuzan)
Loyalty to blood and tradition vs morality(Shanks, Shamrock, Garling)
Only Luffy has a confirmed way to read the RP's, and probably Blackbeard will end up with one with Pudding or whatever, meaning there's only 2 characters that are really in the *race* for OP. Shanks and Buggy also want it, but they'll probably never even to make it to laughtale. Meanwhile, Imu, Sakazuki, Kuzan, Garling, Dragon etc all have an interest in it, but they're never going to really commit to it full-time, they have other goals and zero way of reading the RPs