What i understood was all investors are buying basket of tasty cookies and no one is buying meh cookies' basket or cookies. So tasty cookies are overvalued and meh cookies are under valued.
In both cases the actual buyer is an intermediary, who is managing many people's money. Those people don't even know or care what is being invested in. If for some reason many of them decide to disinvest around the same time (say a recession) they may be hurting themselves due to the asymmetrical nature of the action.
At that point, an active investor can say that any losses by holding the stock an additional month would be eclipsed by selling immediately. A passive investment does not have that ability.