I'm assuming she was flying in first, not a middle seat somewhere in coach.

In that case, she would be very close to the front exits. I guess I'm not seeing how "holding back" the rest of the crowd would help get her off faster.

In case of an obstruction to the front exits, I think they'd be better served by simply getting everyone else off to get them out of the way. Guns or not, there's going to be a lot of confusion and noise and people are going to crowd to the exits. Even if the Secret Service did yell "Stay put or I'll shoot!" most of the passengers wouldn't hear it over the din.