This can be done by beating Mom then teleporting out of the boss room to anywhere else on the floor, or, alternatively, by inheriting any of the items from a previous run. Helpfully, a tinted Skull will always appear somewhere on Depths II which when destroyed drops an 0 - The Fool. If Isaac has Little Baggy it will turn into Telepills instead.
The boss room Dad's Note appears in is representative of the same one that would normally contain Mom if the player were to have chosen the alternative path stages. However, as Mom has been previously killed in Chapter 3, the boss room reflects that state as being completed with blood on the floor and no exit door as would normally be the scenario after defeating Mom.
Before update 4.0.2, Little Baggy caused the guaranteed Fool card in Depths II to be replaced by a random pill. This is now a guaranteed Telepills.
Before update 4.0.2, Sack Head had a chance to cause the guaranteed Fool card in Depths II to be replaced by a sack.