Yeah they’re wrong. Sourdough is fermented with a symbiotic (probs not literal symbiosis, biologists don’t kill me with Mr Killalot’s solution) culture of wild yeasts (some domesticated in starter for hundreds of years now so questionable name) and bacteria.
The yeast makes alcohol and the bacteria makes acids. Consequently the loaf is sour.
you could, and probably someone on this enormous earth does, add sugar and cook before it’s all used to make sweet and sourdough I guess.