He seems to be implying that the naive idea of passive funds of actually holding a basket of the indexed stocks is not the reality and that there are various financial shenanigans going on analogous to the issues seen in the run up to the last financial crisis.

He points to a mismatch between the daily trading volume of various of the smaller components of these indexes and the amount of money globally indexed to them. The suggestion as I understand it is that some of the indexed money is not directly holding the indexed stocks but is using financial instruments to track the indices indirectly and that in the case of another global financial crisis those instruments could break down and you'd potentially see significant divergence between the tracking funds and the actual indices.

That's just my layman's interpretation though, I'm not an expert on this stuff.