Comment by saosebastiao

That is mutable state. Your Redux store is a mutable state container. If your redux store returns a {a:1,b:2}, and then you dispatch an action and it now returns an {a:2,b:1}, then you have mutated your state. It doesn't matter at all if the store's data structures are mutated in place or replaced with a modified immutable updated copy of a previous state, you still have mutable state.