Or maybe he just started excavating and struck the motherlode? Just because their historically hasn't existed silver mines doesn't mean that it is impossible to dig for the metal in the Forbidden Lands. Just unusual.
Maybe? I mean, it isn't impossible but it seems like that would be a pretty big deal. It strikes me as a bit odd that a setting detail like that would be hidden away in a random encounter. That makes me think it may not have been entirely intentional.
I've noticed that there are other apparent inconsistencies in the random encounters. For example, there is the one with an elven family, including children, that was caught and are about to be killed by bounty hunters (also elves). Everything else I've read about elves in the book has let me to believe that they don't have children in the usual sense. I'm also a bit surprised that elves would engage in such cruelty against other elves.