So, continuing with the cave theories, I found a nice person (Xivioke) to let me check out their story mode instance! 8-man story mode yields nothing, but going to explore a bit before checking out 16-man story mode.

Will also stream for a bit :3 Stream closed now :3 But recording up as per usual~

Edit: 16-man Story mode also has no message. It looks to be 16-man HM only. Very interesting.