I believe it's the FURNACE_OPERATOR token which gives them access to available metal ores. For instance, kobolds don't have picks, but they get access to metals because of that token.
No, it gives access to metals IF they have access to ores.
Digganob, I had a thought that because they live underground they might have access to ores by default?
The mere fact of their default site being CAVE_DETAILED does not appear to result in them having access to ore, however, it has just occurred to me that their "liking" regular caves, and therefore inhabiting them often, may result in their having access to metals. However, this too is apparently not the source of their metal.
[ENTITY:SKULKIER]
[SITE_CONTROLLABLE]
[CREATURE:CAVEGOBLIN]
[WEAPON:ITEM_WEAPON_CLUB]
[WEAPON:ITEM_WEAPON_CLUB]
[WEAPON:ITEM_WEAPON_SHARPSTICK]
[WEAPON:ITEM_WEAPON_DAGGER_LARGE]
[PANTS:ITEM_PANTS_LOINCLOTH:FORCED]
[CLOTHING]
[USE_CAVE_ANIMALS]
[USE_ANIMAL_PRODUCTS]
[FRIENDLY_COLOR:5:0:1]
[ABUSE_BODIES]
[OUTDOOR_WOOD]
[DEFAULT_SITE_TYPE:TREE_CITY]
[LIKES_SITE:CITY]
[TOLERATES_SITE:CITY]
[TOLERATES_SITE:TREE_CITY]
[START_BIOME:ANY_DESERT]
[START_BIOME:ANY_FOREST]
[START_BIOME:TUNDRA]
[BIOME_SUPPORT:ANY_DESERT:1]
[BIOME_SUPPORT:TAIGA:15]
[BIOME_SUPPORT:TUNDRA:5]
[BIOME_SUPPORT:ANY_TROPICAL:15]
[BIOME_SUPPORT:ANY_TEMPERATE:15]
[SETTLEMENT_BIOME:ANY_DESERT]
[SETTLEMENT_BIOME:ANY_FOREST]
[SETTLEMENT_BIOME:TUNDRA]
[SETTLEMENT_BIOME:ANY_GRASSLAND]
[SETTLEMENT_BIOME:ANY_SAVANNA]
[SETTLEMENT_BIOME:ANY_SHRUBLAND]
[BIOME_SUPPORT:ANY_FOREST:1]
[BIOME_SUPPORT:ANY_GRASSLAND:1]
[BIOME_SUPPORT:ANY_SAVANNA:1]
[BIOME_SUPPORT:ANY_SHRUBLAND:1]
[BIOME_SUPPORT:ANY_RIVER:1]
[BIOME_SUPPORT:TUNDRA:1]
[PROGRESS_TRIGGER_POPULATION:1]
[PROGRESS_TRIGGER_PRODUCTION:1]
[PROGRESS_TRIGGER_TRADE:1]
[PROGRESS_TRIGGER_POP_SIEGE:3]
[PROGRESS_TRIGGER_PROD_SIEGE:2]
[PROGRESS_TRIGGER_TRADE_SIEGE:1]
[ACTIVE_SEASON:SPRING]
[ACTIVE_SEASON:SUMMER]
[ACTIVE_SEASON:AUTUMN]
[ACTIVE_SEASON:WINTER]
[AMBUSHER]
[ITEM_THIEF]
[SKULKING]
[MAX_STARTING_CIV_NUMBER:150]
[MAX_POP_NUMBER:15000]
[MAX_SITE_POP_NUMBER:200]
[SCOUT]
[WANDERER]
[PERMITTED_JOB:FISHERMAN]
[PERMITTED_JOB:FISH_CLEANER]
[PERMITTED_JOB:BUTCHER]
[PERMITTED_JOB:COOK]
[PERMITTED_JOB:HUNTER]
[PERMITTED_JOB:ANIMAL_CARETAKER]
[PERMITTED_JOB:ANIMAL_TRAINER]
[PERMITTED_JOB:TRAPPER]
[PERMITTED_JOB:LEATHERWORKER]
[PERMITTED_JOB:ANIMAL_DISSECTOR]
[PERMITTED_JOB:BONE_CARVER]
[PERMITTED_JOB:CARPENTER]
[PERMITTED_JOB:WOODCUTTER]
[PERMITTED_JOB:WEAPONSMITH] new
[PERMITTED_JOB:ARMORER] new
[PERMITTED_REACTION:MAKE_SHARPSTICK_FROM_WOODBAR]
[PERMITTED_REACTION:MAKE_CLUB_FROM_WOODBAR]
[PERMITTED_REACTION:MAKE_WOODBAR_FROM_WOOD]
^ The raw of the entity, minus ethics, laws, and banditry for brevity. Removing any required reactants from the woodbar raw seemed to have no effect as well.
Given it's not an issue of location, or an issue of the existence of miners or furnace operators within a civilization, I have no ideas at this point where the issue may lie. I had thought before you came along that this was now a universal bug, which prevented any civilization from having only one metal which they used, but you have proven it's entirely possible.