I wouldn’t read too much into it. First and foremost, Solvang is a big tourist trap. This “Danish” town prominently features, for example, a store famous for its collection of German cuckoo clocks and a museum of mostly British motorcycles. You’ll no doubt find Delft plates for sale there for the same reason you find T-shirt shops; it’s a cheap souvenir, and most tourists stopping through on their way up the coast won’t be too picky culturally. It’s the same reason you can find Japanese lanterns for sale in San Francisco’s Chinatown.
Though the Netherlands is more famous for them, Denmark does have its windmills and wooden clogs. The first windmill in Solvang, built in 1947, was designed by Danish-American architect Ferdinand Soresen in the Danish Provincial style; he built another in 1950 and Danish immigrants Borge and Mimi Andresen built another in 1957, so I don’t think their presence can be ascribed to Dutch influence. Many buildings in the town center were retrofitted with false timbering and thatched roofs to appeal to tourists, and not originally designed in the Danish style— in fact, like most of California, many of the older buildings were originally Spanish Mission Revival style. In other words, all you see— whether it looks Danish, Dutch, or something else— is modern-day aping.
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