Sourness of sourdough

I was watching a video about making Panettone using a natural starter, the way it was traditionally done. They showed a very stiff starter that is wrapped in cloth and then tied with string which the man said was to prevent the starter from becoming too "sour" which would overwhelm the flavor of the Panettone.

He feeds it the normal 3 times before using it which I thought would minimize the sour notes of the starter. Anyone have more information behind this practice?