If you follow the links in the article, you can get to the actual curriculum itself. Scroll for a while and you'll find this...
SS.68.AA.2.3 - Examine the various duties and trades performed by slaves (e.g., agricultural work, painting, carpentry, tailoring, domestic service, blacksmithing, transportation).
Benchmark Clarifications:
Clarification 1: Instruction includes how slaves developed skills which, in some instances, could be applied for their personal benefit.
And frankly, it's not inaccurate. Slaves who worked as artisans were sometimes able to make money independently and even buy their own freedom from masters who allowed it. The absolute atrocity of the triangle trade and chattel slavery is not diminished by a lucky few, of course. The benefit of the skilled craftsmanship of enslaved people overwhelmingly went to their masters.