The funny thing about dead people is that their understanding of time is rather warped. To someone like Hornbelly, a minute might as well have been a hundred years, but the wounds and injuries that the Rockseekers had left on his mortal coil still feel fresh. Ultimately, he's utterly confused by Mista attempting to tell him that the Rockseeker he seeks died a whole number of years ago. There's not a whole lot of time for this interaction to go down, considering the state of Maru and what's going to end up happening to it.

But he ultimately understands. He thought those Rockseekers looked different. His mind is starting to lose some of its sharpness, and he knows that.

He asks one last favor and it's a two-parter. One, is to take the remains of the note and bury it in Phandalin to put his soul to rest. The other, is to check up on the descendant of Julius Neverember and see if the augmented staff wound up working to allow contact with the Elemental Plane of Water. He tells you that you're off the hook, and he won't be haunting you anymore, but it's a request from an "old dead miner who watched a forge for nearly a decade."

Whatever comes out of this for Mista, he probably won't be able to communicate with Hornbelly after he's laid the letter to rest. It's more of a personal mission if he decides to take it, at that point.