Is "hacking small planes" a separate or larger problem from/than general small-aircraft security?
It sounds like just another theoretical scenario.
If that "hacker" wanted to do harm, he could also loosen a few bolts, disrupt the engine timing, plant a bomb, you name it.

Sure, in today's world of aircraft loaded with electronic systems the operators have little knowledge of and trust implicitly, it's quite possible that something that deliberately feeds wrong data to those systems goes unnoticed than disrupting the cylinder timing or mixture control cables would have in the past, but it's still a highly remote possibility.

What it sounds to like me is someone taking the scare of "hackers" breaking into your car and taking over the engine electronics to the next level.