What the author is taking about is tactical asset management, or market timing, which the data shows underperforms a fixed asset allocation strategy. Of course ETF and index funds do not hold cash as they are designed to track the market. During crashes, many investors will panic and sell, whether they hold individual stocks or ETFs. It's not the structure of ETFs that's the problem, it's the behaviour if the investor. The solution, however, is not to hire an expensive active management doing tactical asset management. That doesn't work. The solution is to stay the course and rebalance your portfolio as needed.