In that instance its used as a buzzword just to sell a holster. Period. you are right it wasn't stamped with nails, but its a copy of one that may have been.
I've heard the term used in folk art, basically it came from people using whatever metal they could scrounge to create tools for their project nails being the most common, Scrimshaw for example is nail art, so is some metal engraving. the old tin lanterns punched with nails and made from cans is another. i have a great piece of nail art, my dad after ww2 was a rough neck, he made a graver out of a nail to tool his aluminum hardhat with all his travels, pictures and such. Leather work was no different holsters, saddles and about any leather was ornamented at some point by people with the very basic of skills and tools. Its all under the heading of folk art. You want to see it go to a museum only the best examples go in books. I've seen a ton of it in museums, i love that book but its only a few holsters out of hundreds of thousands so limited in its knowledge base.