He'd been in a lapse between groups of enemies, somewhere after losing sight of Kanji, when he heard Adachi speaking. Well, clearly it's meant for someone else, basically anyone but Jack, but there is Jack hearing him while the other person does not.

Jack has never been good at plans, but this one comes to him very quickly and leaves him very proud of himself. If Xavier isn't watching over them, and everyone's distracted by U-Men... he can beat up Adachi and maybe not get in trouble! It'll be his word against Adachi's! Yes, brilliant. Fontaine would be proud.

So maybe Adachi was going to help, and maybe he wasn't, but what he gets is a certain six-foot toddler grabbing him by the shirt collar and punching him in the face. Because Jack just doesn't trust his ability to get any punches in if he doesn't act fast.