Pit's greatest advantage in the Rosaluma matchup is one that the other swordsmen share - Rosalina's easy to exploit build. Rosalina is light, very light. That coupled with her tall frame gives us an extra edge when facing one of the strongest characters in the game [so far]. Because of our generous disjoints this game, Luma suddenly becomes less of a disjointed wall for us to slog through; openings to take advantage of start to appear.
This doesn't take that much away from Rosaluma however. Rosalina has disjointed neutrals of her own, and can space by her herself quite well - Luma only exarcerbates this problem. Our arrows require more precise aiming because of Luma and, unlike Dark Pit, our SideB does not take him/her away from the picture the moment it hits. Pit needs to properly place his pokes in this matchup more than usual lest we get smacked by galaxies. As was said Guardian Orbitars can be a powerful tool to reflect the repositioning of Luma through Luma Shot, but at competitive levels the Rosaluma player will seldom use said option. Power of Flight is a wonderful recovery, but is also easy to gimp once the trajectory is traced. This is problematic when we are facing a character like this that can either choose to precisely position hitboxes to deny us sweetspotting the ledge or to follow us off-stage with a good edge game and effective recovery.
On the other hand, Rosalina's recovery is somewhat predictable as well which gives us a good window to gimp her - with Guardian Orbitars given the chance. When delving into customs we can also find two additional tools for facing Rosaluma. The first, Piercing Arrows, gives us a projectile that won't care if Luma is in the way or not. Impact Orbitars is then a second option, and as @Makorei mentioned a good tool to get Luma away, albeit risky. Rosaluma's most notable upgrade from customs is Luma Warp
Overall the matchup feel even, 50:50, but given the relative safety of our attacks I could see this at 55:45 Pit's favor as the meta for these characters progresses. Rosa punishes hard, but I do not see her zoning us as roughly as she does so to other characters.