They were already used as a tool, and so can't be used further (due to irrational fears of nominally negative interest rates). Printing money is effectively equivalent to lowering rates.
“Printing money” isn’t a literal description, its a metaphor for a bunch of other “loose money” monetary policy choices which increase the money supply including, not alternative to, maintaining low interest rate targets (in fact, most of the other actions are the means by which actual rates are kept near the target rates, rather than actions in addition to acheiving rate targets), which is an ongoing intervention.
