Continuity mistake: During the "Midnight Express" scene, Chip opens his shirt and presses his chest against the glass. The shot moves to Steven, and as we pan back to Chip, he is sitting, phone in hand, with his shirt buttoned. (01:09:05)
Continuity mistake: When Chip's talking to Steven through the glass, after he stops talking about the Midnight Express, you can see he has no phone in his hand yet when Steven's telling the guard to arrest him he has the phone in it. (01:04:05)
Continuity mistake: When the cable guy puts chicken skin on his face, the bit over the right side of his forehead disappears between shots.
Answer: If we're to assume it to be an intentional addition to their dialogue, and not just a minor continuity error, maybe it was just Steven's way of handling Chip's uncomfortable initiation of the topic? I can buy Steven not understanding what Chip was talking about when he said "I set em up, you knock em down" without much context to set up that convo. But once Chip says he juiced her up and helped get her back, it seems like Steven gets what he's hinting at. The way he turns away and sort of flees while asking "how did you know we're back together?" (to me) seems a little more like an effort to avoid acknowledging what Chip did, and a little less like him genuinely not knowing Chip's involvement. As if he was in denial and didn't want to openly accept the situation for what it was. That's just my guess anyway. I hope it sounds like a fair enough point and not like something I'm looking too deeply into. Never really gave that moment in the film a second thought until reading the question.