As a security consultant, part of my job involves ethical hacking and supervised penetration attacks on client sites. I've only ever done this in financial institutions; I've never tried to hack a plane, and, to be honest, my hardware and software experience aren't up to that task. I often need the assistance of a hacker of systems at the desktop layer in order to then penetrate the underlying systems.
A couple of observations:
Professional hacking is, of course, done in a very heavily supervised manner, with every step documented for subsequent analysis and mitigation.
Professional hackers never ever attempt to hack systems without invitation and without an appropriate contract in place.
Professional hackers never disclose their findings to anyone other than the client. Any findings are the sole property of the client. To successfully penetrate a system and to broadcast the fact would directly compromise the client's security - and our job is in the opposite direction.
There are a lot of idiots out there making wild claims in an effort to gain some notoriety. There are also dedicated professionals who quietly and without fanfare are paid to expose vulnerabilities in order that these can be eliminated.
I doubt that he has hacked a plane's systems. I wouldn't be certain that it can't be done, though.