The thermal connection however, comes from the rubber seal around the heatsink. Those do touch the lower case, therefore can transfer heat.
However, since the heatsink would be 60 deg C and the lower case being only a cool 20-30 deg C, it is possible the heat is transferred through the air, rather then a physical contact (which would better for heat transfer). If you were to add mass that actually makes physical contact with the L bracket to the lower casing (no electrial contact since the case is lined with a piece of plastic? or something like that), the temps would drop DRASTICALLY since there is 10x more mass to move the heat to. (remember heat moves to a cooler place, trying to balance)