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.
"It" did actually happen, shortly after the post, and was therefore both imminent and likely.
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.
The jokesterism is in response to a joke world. There are innumerable numbers of murders each year but somehow a spree killing is enough to slay irony? 9/11 wasn't enough to slay irony. The reaction to it excaberbated the ironic mode of moderm culture.