I have not heard of anyone yet hacking DJi's code, maybe I am wrong and it can be done but nothing on the forums i have seen.
You run the risk of bricking it. I have seen articles of GPS jamming and false GPS signal injection but no hacks yet.
Yes you can build your own with a pixhawk etc, hence i am sure that the drone involved in this episode was a home brew. Do the other drones have the legs or range of DJI's machines, I am thinking of lightbridge, and its a guess the the drone operator in this case wanted to see/film where he was flying (aircraft landing) via his video downlink.
Yunecc's video link is reported to be only 500m line of sight were as DJi can go for a few km.