Based on multiple things, my interpretation is that 1) the white butterflies belong to Carlo; and 2) the entire DLC is about P getting to honor what Carlo truly wanted, after everything he's been manipulated into doing over the first 8 chapters of the basegame. It's like finally giving someone a proper burial, after they had been brutally desecrated.
But what the white butterflies actually are isn't important to me. What matters to me is what they meant to P, what he thought about them, what he got out of the events of the DLC.















