My understanding of the deep stall was as the a/c is sinking the relative airflow now comes more from below the wing meaning you can have angles of attack of around 20 degrees with 0 degrees pitch angle. This is then exacerbated by the nose up moment from the centre of pressure moving forward.
The T-tail can make the deep stall impossible to recover from due to tail blanking, where the wash from the wing also disturbs airflow over the tailplane. This would make pitch inputs difficult/impossible.
Anyone please feel free to correct me if my understanding of this is incorrect.