China is the greatest exporter of Granite to the United States. When you go to vendors anywhere in the US, chances are the slabs have come from China. Costs would not be driven down even if it was possible to purchase a Chinese quarry(or mountain) and have your own labor force(JWs with quarry experience in all phases? NOT LIKELY) preparing the stone for export.
The story as presented is complete rubbish.
More likely, WT purchased the equivalent of a "mountain" in granite from a US vendor.(WT could easily set themselves up as a vendor) And like many stories coming from the R&F it has been embellished and amounts to a fairy tale.