It's an interesting story, and though the subheading says the treasure hunt was unsuccessful, the text implies the treasure may have been found already. The entire story is a reprint (verbatim) of one that appeared the previous year in a Springfield newspaper, and appears in other places as well. It seems like a piece of fiction (no names of the important characters, though some minor characters are named, but those names all lead nowhere).
But... I did find one other reference to what sounds like the same legend in 1895, but it is also vague and describes events decades earlier, and again with names that lead nowhere.
The only earlier mention I could find of a robber's cave in Stone County was in 1891, when supposedly there was a standoff at a cave with some bandits holed up inside, discovered by some "prospectors from Colorado". But that appears to be a different story altogether, though it does at least put a "robber's cave" in Stone County. Here's the info:
And then the followup:
But there are so many caves and legends in that area so it's pretty hard to follow, even if you've got boots on the ground.