It is a matter of the level of security, I guess. You, too, could throw a rug over a fence topped with razor wire to get onto the secured area of most any airport. You might even be able to get into an aircraft, but without specialist knowledge that might be as far as you get. That said, there was the 'Barefoot Bandit,' that goofy kid who stole numerous aircraft until he was finally caught.
I don't know about this particular type of aircraft, but I happen to have two keys in my possession that would enable me to unlock two fairly common types of aircraft, one 'large' and one 'small.' All of the aircraft of each type share a common key, and I have the keys because sometimes I flew those types.
Then you have the way that many small airports are secured by a keypad with the combination known to all, if not simply written down next to the keypad. All in all, we still depend on the fact that almost all of us are reasonable people who do not plan to steal an aircraft.
The odd lunatic might be deterred or simply delayed by what we have for security but he would not be stopped. That would mean posting armed guards on each and every parked aircraft, plus finding a way to read the crew's mind before each flight.