I mean he abandoned them and joined a Yonko crew.
Even if he’s secretly a part of SWORD, they’d still need to give him a bounty, just like they did with X Drake.
And to be consistent given how strong he is, it’d need to be MASSIVELY higher than any first commander’s bounty, comparable to the Yonko bounties.
Also I’m gonna assume you meant 900 million, not 900 thousand, since even East Blue Luffy had a bounty of 30 million, and there’s no world in which anyone thinks Aokiji is worth less than East Blue Luffy.