In 957, Brannew said that no pirate has bounties surpassing those of WB and Roger. But I find this very strange. Because Xebec should have surely had a bounty on the level of WB and Roger's.
As Sengoku stated, Xebec was Roger's greatest rival. This inherently hypes him above WB. So does the fact that Xebec was literally WB's captain.
I think that its possible that Xebec didn't have a bounty. That basically, he was such a threat to the WG, that there wasn't even a point in granting him a bounty. Instead perhaps there was a different reward granted to one who could kill or capture Xebec.
We know that the Gorosei can offer someone celestial dragon status. What if this was the reward for killing Xebec. This would be the ultimate reward the WG could grant an individual, hence I think it makes sense for it to be the reward they'd grant for someone like Xebec.
Xebec didn't simply want to find One Piece or just chill with his family like Roger or WB, he wanted to be King of the World. Dude was basically trying to steal Imu's throne, not that he ever knew of Imu's existence. Hence I could see Imu granting such a high reward to anyone who would dare to challenge him.
Also just looking at everyone on God Valley.
On one side you had the rocks pirates, and on the other you had the Roger Pirates, Garp, the holy knights, and Saturn.
While we don't know to what extent Saturn and the Holy knights participated in the god valley fight, if Rocks was attacking the CDs, we can presume he fought them. This should show the extent of the forces it took to stop Xebec.