Check out this photograph of Trump exiting his limo:

First of all, look at the thickness of that door. Look at the agent on the right opening the door. It looks like he's exerting himself a bit, his hand gripping the upper door frame. Look at the width of that upper door frame.
The lower section of the door frame looks like it is connected to an articulated arm running under the frame. I'd be guessing here, but the when the door is closed that articulated arm further connects the door to the bottom of the frame, giving an additional connection from the door to the frame in the event of an explosion.
For comparison, this is a car door frame if you are not President:

So I'd say that the door on that limo, as well as the unpaid agent opening it, are both defending the President.