Dressrosa was like the day after Punk Hazard though, it sounded like people knew about the ten titanic captains for a long time.
even though it seemed like what you've said, but that's what actually happened, his joining was known to the public during Dressrosa arc, & there are two evidence proving this:
- Gorosei said that they are shocked that someone as strong as Aokiji joined BB, they said that after Doflamingo was defeated.
- Burgess telling BB that he doesn't trust Aokiji just before his match, & right after that the commentary says that there 10 BB commanders, most likely Burgess & that commentary read that day's news paper & knew about Aokiji's joining.