I think the point is that they don't have the influence to intentionally deviate from the index because they think they know better. If your mandate is to be passive, then you need an index to follow. If you are that sure the S&P 500 index is wrong for some reason, or whatever other index you follow, then you need to invent a new index. Then, you can follow the new index.
replyAt least my index funds do that. They don't get to constantly trade like non-index funds do, and they typically stick with the index, but every index fund I own has a line about "we select stocks that we think will match the index", which is different from buying the stocks from the index.
replyAgain, the vast majority of the time they are matching the index stocks. However they have the right.