Captain Morgan has stated his Operations Manual (a legal document) puts him responsible at the moment he/she "arrive on board" and this indeed will then be a legal position in a court as it does not override any government agency regulation and is in fact approved by one ore more.
But Captain Morgan does not have free rein and must still follow instructions/ orders of other personal like the D & A tester, Airport instructions such as push back and start clearance even a local cop that wants to make a last minute arrest on the aircraft.
He/she can not deny any of their requirements while at the gate.