You absolutely get it. To Kat the punch means next to nothing. The only reason she's doing it is because she owes Mac from leaving her to the cops. Really getting her gun back is more important to her than hurting him, but she's not entirely bad, so while he's taking care of Marie, she'll let him keep it.
Yes, the person with Kat was just a random person, to be specific, her new girlfriend Amanda, though, she has no real part in the story, just wanted to add that scene to show how little the entire event meant to her.
She could simply move from decking Mac, right back to her date, or whatever they're doing seamlessly without any hesitation.
Seems like you've definitely picked up on Kat's character pretty nicely.