When investors sell that amount, it doesn't matter whether they hold the underlying assets directly, or via index funds or ETFs, or via actively managed funds. The market will go down. So, which part of the problem is uniquely due to index funds?
Burry hasn't made that point very clear.
There might be issues with (liquid) index funds that give exposure to inherently less liquid assets, such as bonds or real estate. There might also be issues with index funds that do not hold the assets themselves, but replicate the exposure synthetically by entering a swap with a third party, giving rise to tracking error, counterparts credit risk, etc.
However, as I said, Burry hasn't enunciated these concerns very cogently (at least in the extracts quoted by Bloomberg). This article here does nothing to address those concerns.