Earlier quoted context omitted.
My understanding is that in general there no need for supplements with a normal, balanced diet, especially with eggs, dairy products, grains, and others if iodine is naturally present in the environment. So if Switzerland imports a lot of those, raw, or in prepared/processed food, or even the animal feed for its hens and cows the Swiss today probably already get much more iodine in their diet than 100 years ago.
People have been struggling to get enough iodine for a hundred+ years. That’s why it’s added to salt. This isn’t a 21st century problem.
But the point is that Switzerland's environment is especially poor in iodine hence the specific health problems it used to have, and which were much less serious in neighbouring countries.