The fact that they return a default value (or they have a default
argument value) doesn’t make them reducers IMHO.
Reducers do not just return default values. They always return the accumulation of the state (based on all previous and current actions).
Therefore, they act as a reducer of state. Each time a redux reducer is called, the state is passed in with the action (state, action). This state is then reduced (or accumulated) based on the action, and then the next state is returned. This is one cycle of the classic fold or reduce function.
As @azium summed up with state -> action -> state.