You are misreading my post and quoting the wrong section. For a normally flowing shot, extraction is determined by grind setting, nothing else. The order of extraction is as follows:
Redbird has almost no acidity; it also uses a fairly rough Brazil that cups bitter and astringent. You need to extract every last drab of caramel to give it the chocolate tastes its proponents value.
The overextraction listed on the graph is a flaw: you can extract too far and get a blah drink with too many caramels. Overextraction on the other hand tastes like instant or cooked coffee. This is not very likely on a DB, since the shot doesn't cook if you choke it.
The balance between bitter and sour is basically set by the coffee and roast; and in Redbird, it's almost all bitter. You can slightly tweak this bitter/sour balance by changing flow or temperature, but more and more I'm getting the sense it cannot be done nearly as well or reliably as tweaking the (bitter+sour)/sweet balance by manipulating the extraction.