Cultural: As we saw in Helaman 3, Mormon is not necessarily historically careful in his attributions of cultural items to this land northward. Indeed, he has a reason to what to define the land northward in terms of his current world rather than this world as it existed at an earlier time. It may be that we have a similar process occurring here, as widespread metal working is later than this time period in Mesoamerica. This statement would make more sense at the later time period where Mormon previously focused on a city of cement.
In any case, we should remember that the economic value of gold and silver in Mesoamerica lay more in the items crafted than in the material used to craft them. Gold and silver were typically not valued in and of themselves, but rather for the things which could be made of them. Thus this statement that there are workmen working the ore accurately suggests the process of production of trade goods. It was the movement of these unusual and prestige-related goods that created the wealth mentioned in this verse.