This article is bogus and misleading.
In a crisis market the bond market is so huge, wide, that unless the fed did some underlying buying, mark to market would turn the entire system into a dumpster fire.
-28 yr finance vet
Quite true. To avert a cascade of defaults, panic, and then market collapse, the Fed has to buy widely. Distortions of price are a lesser evil than losing market function entirely and suspending the ability of credit worthy cities and states to finance their capital needs and of bond holders to sell to willing buyers.