It's not just about defaulting on debt, more about using it to stimulate the economy.

https://www.brookings.edu/blog/up-front/2020/06/05/what-is-yield-curve-control/ This article is quite interesting in explaining how it actually reduced all the bonds they had to buy after implementation (until the current mass sell off)

There's a lot of game theory involved and now the tides are turned, this article is barely two years old and quite amusing everything is different now.