There's "proof" regarding the Double Loot event on the answer I posted in the question's comments. I also think it can be proved based on the Leaked files, but I can't check right now. Furthermore, it matches with what we know about the game. We know the creatures from this raids are regular creatures (it's the same Orc, same race id, not a special Orc), otherwise it wouldn't be possible to activate prey on them. And we know that you can activate prey on them because there are several screenshots of players killing these creatures and getting the loot prey message (but not a double backpack).
It's one of those things we can hardly prove because we can't prove a negative, there's no way to prove it never happened or never will unless CipSoft says so. But even then considering how long it's been with the prey system and double loot events we would have seen a double Backpack drop, which we never did.