React - unmounting - this.intervalID statefull?

** https://www.codecademy.com/paths/fscj-22-front-end-development/tracks/fscj-22-react-part-ii/modules/wdcp-22-lifecycle-methods-d6df42ef-50f8-432f-abe1-c542e222afc4/lessons/component-lifecycle-methods/exercises/componentwillunmount **

It is mentioned that the interval id can be stored as this.intervaID, but I wonder if this not should be persisted as part of the state object as ? Otherwhise, I would believe that the current this.intervalID becomes a stateless prop?

What am I mssing as a concept ?

Johan (Belgium)