It has to be both imminent and likely - if you saw somebody posting something like that, would you think they were actually going to do it? Nah, it'd blend in with all the other larping and shitposts.
This was also the second such incident within just a few weeks. Knowledge about Christchurch should maybe have informed these "shitposters'". Can't really claim it's all just a joke after people have died.
That's not how likelihood works.