According to BIKI - https://community.bistudio.com/wiki/ctrlSetEventHandler
"Sets given event handler of given control. The provided function should return true/false indicating that the event has handled this event fully or not and whether the engine should execute it's default code or not afterwards"
True/false seems to make no difference on map control.
You cannot override the MouseButtonDblClick on a map control by returning true or false with a function.
You can override the MouseMoving event on a map control... but you can't use false to not override.