A die struck or cast-in finish would be very easy to remove by the manufacturer through simple sanding and polishing, so the fact that they also sell the set with a high-polished finish doesn't necessarily rule out the possibility that it is applied early in the manufacturing process and then removed. It looks die struck, tumbled and finally matted to me, but like everyone else, I can't say for sure without seeing it live.
Another possibility is that it is applied by laser and then tumbled. It could also be milled in by CAD either before or after casting. People are doing some pretty amazing stuff with CAD and lasers these days. At any rate, it doesn't look as though it's applied by brushing or any type of scratching process to me. It consists of too many uniform and purposeful but random single lines, for lack of a better description.
I've created a similar pattern using acid and resist techniques, but that was long enough ago that I don't have photos. There are many people contributing on Orchid that are far more qualified to offer advice concerning etching than I am, but if you're after creating a single piece, acid etching might be an avenue worth considering.