That was my reaction too: it's not too difficult to notice that the search in Slack is searching chat messages that you have never seen locally, is it? What's more, this feature is not unusual among HipChat clones... Even Discord just launched search.
I'm not sure who the misunderstanding belongs to -- the people at Fast Company reporting the story as it is at the Gawker people the story was about.
Reporters need to get computer literate... and then hopefully go beyond that to become involved in choosing products that do what they need them to do.
Reporters need to realize that understanding the features of a software product can mostly be done by using it, and paying attention to what is happening and figuring out why.
There is a lot of discussion in the security blogosphere about the security of various messaging platforms that are trying to focus on actual security features, and while there is value in the details that they are hashing out, just knowing what software they are talking about is better than sticking your head in the sand.