It can be a bit confusing.
When we talk of an I2S output in the audio library, it's software component which gets audio from elsewhere in the library and send it to the hardware which will ultimately output the digital data at pin 22 (plus the related clocks on other pins).
So this software component which transmits to hardware needs to have inputs where it gets the data from the other stuff elsewhere in the library.
Likewise, a component we call "input" gets data from the outside world and must have outputs to the other stuff in the library.