In the absence of an explicit definition of start the courts will most likely decide that the flight starts from the time the PIC boards the aircraft and the end of the flight will be when the PIC gets off. Actually they might indeed take it back to flight planning on further thought.
The PIC has to make decisions to continue with the flight from the time they clock on. Is the aircraft serviceable, is the weather ok to allow the flight at both the departure and destination, etc etc. Good luck convincing a judge you aren't in charge given the breadth of the responsibilities before the engines start.
Its unlikely a PIC will be able to argue I'm only in charge from engine start. And if they did rule that way would you really want that. Open invite for mgt to take ALL the pre-flight planning and decision making out of your hands. Some airline will go, excellent, we'll do the fuel, flight planning, and hand you an aircraft, you just fly what we give you, no questions allowed.