I'm not sure you are right there. Must admit I can't remember where I read it but I believe the time of arrival is measured at when the doors are open and the passenger is free to leave.

There was an EU261 case where the plane arrived on stand 2h55 later than scheduled but the passengers couldn't disembark until 20 minutes later due as there were no stairs / ramp operator (can't remember which). Compensation was given was paid as they could only leave the plane more than 3 hours late.