And the Super Heavy will never go to Mars. Starship might. But how it launches from Mars is entirely speculation. For something like Starship to launch you need plenty of pad infrastructure anyway: fueling, chilling, etc.. If you can build that you can build a diverter no problem.

Also on Mars you need to dig anyway to create any kind of habitable environment due to radiation and you have no issues digging caused by the water table. So digging a diverter on Mars might even be easier then digging one at Boca Chica.