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I didn't look at the full context, but they might mean that when backtesting a strategy using the SP500 universe you must not use the current constituents. You have to use the historical ones, otherwise there is of course bias involved.
I guess they might mean that but it’s very much not what they said 
It's hard to read it that way, honestly. They use index prices and say that ignoring dividends underestimates the performance of an investment in stocks but there is an opposing effect: the index overestimates the performance of an investment in stocks due to survivorship bias.
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