The problem is that they cannot copy RB because the rear of the macca is far more cluttered and presents a barn door to airflow by comparison to the smooth flow over the rear the RB has.
They have also failed to fully understand what RB are doing with the low exhaust aerodynamic effects.
Simply attempting to 'blow' exhaust into or over the diffuser has resulted in out of balance DF. The low exhaust has far wider application to the cars rear airflow than this.