What they mean is that when a company becomes part of an index, like the S&P500, historically, they come in somewhere around approximately rank 500. The S&P naturally results in more buying of winners and more selling of losers, so anyone who owns an S&P ETF will own that stock when its approximately rank 500, and will own more and more of it as it grows to be a bigger part of the index (higher rank).

Now, however, these IPOs are so big that straight away they're at like rank 10, or 15 or whatever. The investors who own the S&P don't experience any of the upside when they grow from rank 500 to 10. They are only exposed to the downside (if they lose value/rank/fall out of the index) or the marginal upside if they continue performing and generating returns.

The point is that given the size of the IPOs relative to other companies, their addition to the index has a much more downside exposure than reasonable upside exposure.

I'm probably butchering my explanation here, so apologies if it sounds odd.