It's not a material amount vs overall outstanding market and a majority of the holdings are in higher quality, short-dated bonds that will likely be paid at maturity long before any credit stress is seen. Corporate treasurers are not in the risk taking business. They are squeezing out incremental yield from cash on hand.
Actually, a material chunk of that debt is held by big tech.
https://www.theguardian.com/business/2019/nov/08/how-big-tec...