Its literally told in the manga thats the reason their bounty increased, and they don't just eat, they kill/hunt people who have DF. If one of those DF users was a famous guy like Loki, killing a guy like that would be a huge boost of course.
Lakainu gets 5 billion for defeating Aokiji in extreme-extreme diff fight and becomes Fleet Admiral. If thats worthy of 5 billion, then Teach should've got 6 billion for making Aokiji his underling in a pirate organization, which is something Lakainu could never do as we know Aokiji refuse to work with Lakainu.
Obviously Aokiji joining has no effect on Teach's bounty, Aokiji joining was known since Dressrosa and Teach still had 2.2 billion during that time.