I'm not sure what this changes. Do actual terrorists make their plans public? IANAT, but I'm pretty sure they discuss and plan their actions privately most of the time.
Besides, look at what he wrote:
"On my way to blow up the plane (I'm a member of the Taliban)."
If he somehow didn't expect that line of text to get his Snapchat auto-watchlisted, then he's even dumber than originally thought.