The big tip-off that a flourless chocolate cake is actually a custard is the cooking method: the water bath, also known as the bain marie in French cooking circles. It’s purpose is to ensure that custards (and other heat-sensitive dishes) bake as gently as possible, slowly creeping up on the final setting temperature without overshooting and curdling. It works for the obvious reason that the water surrounding the cake can’t exceed boiling temperature of 212 degrees, which it actually never attains, even in a very hot oven.
But why wouldn’t the water the bath eventually boil if the surrounding air is 325 degrees or more? One word: evaporation. As the water in the bath heats, some of it evaporates, causing the surface to cool. The broader the pan, the more water surface area there is, the more evaporation you get. Most water baths never exceed a temperature of about 195 (less if the pan is made out of a thin material like stainless steel or foil). Cover it though and the heat is trapped inside, causing the water to come to a full rolling boil.