Maybe. If you read the fine print they are not. They have the goal of matching the index returns, but they never say anywhere they have exactly the stocks in the index.

Index funds all make active choices and often hold companies not in the original index. They are more passive than a traditional funds that buys and sells all the time, but they still make active choices. When an index changes stocks they can look up the price - but the funds mirroring the index need to make real trades that if not carefully done will change the value of the stocks (and cause the fund to under perform the index), so index funds have plans to prevent this. Compared to a traditional fund an index fund looks passive and there is much much less for the manager to do - but that doesn't mean the managers do nothing.