Frankly, it's complete bullshit. The systems are completely, physically separate. There is no way to hack the thrust from the in-flight entertainment system because they are not connected to each other. The most he'd be able to do is turn on the fasten seatbelt sign.
Is the in-flight entertainmeny system able to show that world map with the cute little plane that indicates the planes position?
I doubt they have someone copying the updeted position from the avionics system to a USB-stick, unplug it from there and plug it into the entertainment system to update position data every few seconds...
There goes your "completly, physically seperate".
Heck yeah, it's trivial to make such a connection reliably one-way only, but even then, "physically seperate" would be an outright lie.