I'm fairly certain that Aokiji is a double agent secret sword member, and is throwing against Garp.
Aokiji is giving Garp a way to save Coby and scram, hence not going all out.
This could tie well into why BB would keep Law alive, since he would need a new bargaining chip to get king status from WG, since Coby would escape. Since otherwise, there would be no reason to keep Law alive, BB could instead steal Law's DF ability.
Law would be that bargaining chip to leverage against WG/marines.
WG placed 5 billion price on Law's DF. Plus he's an ex-Shichibukai. There would be value in getting Law, for WG