Isn’t duration a type of Interest Rate Sensitivity? Or does the IRS measure refer to price changes from both level and twist changes? The book didn’t seem all too clear on this one

Duration is a type of interest rate sensitivity. Interest rate sensitivity or interest rate risk covers duration, key rate durations/yield curve twists and convexity.

interest rate sensitivity is the generic response of Fixed income security values to changes in interest rates.

Duration could perhaps be thought of as a subset of interest rate sensitivity measures that deals with small parallel shifts in the yield curve.

That is why if we only use effective duration as a measure of interest rate risks, MBS would be hedged using a Treasury security, making MBS market directional.

The 2-bond hedge, which is based on IRS, removes market diretionality of an MBS by accounting for both level/twists