In addition to cutting interest rate to historically low levels, central banks around the world provided ample liquidity at a higher frequency and with longer maturities to a larger group of financial institutions and against an extended list of collateral, agreed on foreign exchange swap lines and entered new territories by buying assets on secondary markets.
If you look at the risk premium of these assets and compare it with the level of interest rates, you can see that it isn’t overvalued. And in the bond market the natural rate of interest has fallen as a result of technological changes, demographics and low growth. The context of this market has changed.