They didn’t overreact, because she’s still alive. And lucky in that.
In the instant that an officer notices her “artwork” on her hoodie there is but a mere flash of a moment to decide what to do. He doesn’t have *time* to carefully inspect the circuit board, wiring and battery and assess whether it is really a bomb, or just looks like it might be one.
That he didn’t instantly opt for a head shot is really lucky for her, but it was a huge risk taken by the cop on the chance that ~maybe~ it wasn’t an immediate threat. So he was lucky too.
The next cop in a similar situation may not be so lucky.
This sort of display could easily be used by a real terrorist as a distraction from the activities of an accomplice who’s about to set off a real bomb, or from the fact that the same terrorist also has a real bomb or real bomb components in addition to the fake. Zero tolerance for this sort of thing is appropriate. There’s no need to wait for a bunch of other people in the terminal to notice it and get scared, and say to a friend “That isn’t a bomb, is it?”, only to have someone overhear that and start panickingand repeating “Theres’ a bomb!”.
Same for the crazy guy who ran screaming down the aisle of a plane that was about to take off (according to a couple of passengers, including something about a “bomb” in the course of his screamings), and got shot dead by an air marshal. It was appropriate to shoot him first and find out what he was really up to later. His wife knew he was mentally unstable and falling apart at the prospect of the flight, and pressured him to get on anyway — all without bothering to alert the gate or flight personnel of his mental condition. Imagine if we just let people do this. How long would it take real terrorists to stage something similar to set up a distraction, to enable them to do something that will result in disaster?