The smartphone aspect is a bit of a red herring. He used an smartphone with an external radio. The same exploit could be done using a laptop, a tablet, or just a box of electronics.
Given that this is 'spoofing' radio messages, there's also absolutely no reason why an attacker would be on the plane. Easier for them to sit on the ground and do it remotely.
Still, it's an interesting article/presentation and nice work.