I think that if Saturn was Luffy's enemy in this arc, then Oda would have built the Luffy vs Saturn fight a little bit more. As someone said, Luffy doesn't even know who Saturn is, he doesn’t know what a gorosei means, he doesn’t know what he did to Kuma and Vegapunk, and we are already at the final stage of the arc with the ticking time bomb activating. That is like having the smile gas spreading or the bird cage activating while Luffy doesn’t know who Ceaser or Doflamingo were.
Either there's no major battle and the plot ends with the strawhats running away, or Luffy fights the enemy who he has a lot more build up with.