Isso é explicado na novela oficial do filme . O Coringa sobreviveu ao acidente de helicóptero exatamente da mesma forma que Harley, saltando dele antes de atingir o chão.
Then hell plummeted from the sky in the guise of a crippled, out of control Chinook helicopter that skidded across rooftops and shredded its way down Ninth Street, only to careen into the bank’s facade, destroying glass, stone, and mortar before its sturdy all-steel vault put an abrupt stop to the aircraft.
The copter was on fire. Nothing inside could have survived.
From across the way, on another rooftop, the Joker watched the flames rising into the sky. He had jumped just in time. If he had waited even another ten seconds, he would now be little more than ash.
Claro, isso estraga a grande revelação final do filme, que é presumivelmente porque eles tiraram essa cena do roteiro de filmagem.