Thanks for posting the link. However, I am not sure why the dry yeast is used. If it is IDY, 1/8 teaspoon for a half cup of flour comes to around 0.60%. That is a lot of yeast. The hydration itself, based on the amount of water in the milk (whole milk is about 88% water), comes to around 176%. The yeast will be quite active in such a liquidy mixture, and I would think that there would be a lot of bubbling activity, but I would think that it would use up most of the simple sugars (the natural sugars extracted from the flour and those derived from converting the sugar added to the mixture to forms usable by the yeast) and deny the wild yeast a better share of the simple sugars. In due course it is possible that as the mixture takes on a more acid composition the commercial yeast will die off and the wild yeast will take over. Maybe that is why it takes 12 days to get the starter in workable condition.
Maybe some other member who has a better understanding of these matters than I can explain the reason for the commercial yeast.