Index funds generally try to match the performance of the index, but most are not required to hold the same companies as the index itself does. They typically do, but managers often have choices.
reply> Index funds already seek to own the entire market, [...]
replyNo, it depends on the index in question.
Yes, most index funds don't literally intend to own the entire market for any sufficiently broad interpretation of the word "entire."
replyBut the point is that we have notable index funds which are marketed to customers as having the intention to own segments of the market according to certain rules, and they are changing those rules with relatively short notice and for reasons that seem suspicious to many customers.