It looks like they are boiling the potatoes in most of those recipes, so they are actually culturing yeast from the air in their kitchen rather than whatever yeast was on the potato to begin with. You want to be careful with root vegetables and this kind of culture. Non acidic foods such as potatoes can grow Botulism. It takes pressure-canning at 250F to kill botulism spores, so simple boiling will kill most yeast and bacteria on a potato, but not botulism spores, which can be present in soil. Wort is acidic enough to prevent botulism form growing, but a potato is not. You could probably culture some wild yeast on a plate, or just grow a yeast culture with a known strain, just be careful with growing things in non-acidic foods. Heat will break down the toxin, so you won’t get poisoned from eating a cooked potato, but using a yeast culture like this without cooking the end product (beer vs. bread) could be risky.