Like most physicists, I have zero formal experience in this. It honestly doesn't feel like a topic for a physics journal even though a reliable positive result would have profound implications for physics. In the same way, I wouldn't expect a technical article reporting measurements of carbon flow in the environment to be published in a political science journal, even though climate change might have profound implications for global politics. [Discussion note: the validity of climate change has nothing to do with my point here.]
All that being said, I also don't find their negative result to be particular compelling. If their two search terms were "Pope Francis" and "Comet ISON" as described, well, Comet ISON was a dud (and thus won't be much of a topic in the future), and it's easy to imagine scenarios in which future time travelers would not have a particular interest in Catholic history. And that's quite apart from the possibility that time travelers would make some modest effort to avoid asking their neighbors about future events. Maybe that's all accounted for in their paper in a compelling way, but I honestly don't see any way they could ever argue for more than a very limited negative result. And I'm not convinced that's publishable in any sort of competitive journal.