Power is 100% fucking with Makima. As much of a selfish bratty little sister she is, she has routinely had Denji's back (after a point). I can't remember when it was, but I remember her pulling a stunt like this before and using it as an opportunity to lower the opponent's guard for a sneak attack. Not that she's super loyal or anything, but handing over Denji to Makima is the worst move for Power and on some level she knows that, because Makima killed her once for her own purposes and she'll do it again.
That or someone else is about to jump in and try to kill Makima and Power will immediately switch to their side since they're more convenient to her, making it so that Power's actions ultimately served to buy the time needed for Denji to make it out alive. There's no way that we'd be given the narrative equivalent of a ruined orgasm by having Power come back and be completely pointless to how things play out, as much as Chainsaw Man has fucked with our expectations and emotions all of that carried impact, progressed the story, and worked. If Power offering Denji up ended up how Makima wants it to, then narratively we'd be in the same place as if this chapter just flat out didn't happen.
