It probably wasn't an ACoC clash. As people pointed out, the same black lightning Release happened when Ulti and Luffy or Lucci and Luffy clashed. We haven't seen either of them coating their attacks in CoC, which is illustrated by a trail of black lightning surrounding the fist. Compare it to Big Mom hitting Page without touching him, or Luffy punching Kaido without touching him. Or take Garp's Galaxy Punch that was illustrated by a trail of black lightning flowing out of his fist, upon obliterating the town with a shockwave.
It means Garp in that exchange of punches was more restricted, as we know the scope of Garp's punches, while Aokiji is not a confirmed ACoC user. He may be a regular CoC user though; we know that regular CoCs colliding also creates a shockwave accompanied by black lightning, although with touching.
If Oda wanted to illustrate Aokiji being an ACoC user, he did do a poor job.
Nevertheless, Aokiji being a brawler that exchanges blows with Garp was impressive, taking Garp's punch straight to the face. Albeit it was an old rusty Garp with his abdomen stabbed.