As others have said throughout these comments, as long as you keep the temperature high constantly, it won't be an issue.

Bacteria won't survive in simmering water, so time alone will not cause it to go bad. The toxins that bacteria produce can survive, which means tainted ingredients being added might spoil it, but as always, the poison is in the dose. With a large enough pot, a couple pieces of spoiled food likely won't cause illness.

I think if it is brought to a boil every hour it should be safe-ish

how do they manage to avoid that in the article? they list at least three different restaurants that serve these long-running soups, surely they don't poison their customers?