If 90+% of a journalist's job is to open documents from sources in an office program, what good does it do to cage exploits in to LibreOffice? Those exploits still get virtually all the data an attacker wants from the journalist.
The data still has to get out though. If LibreOffice doesn't have network access then that can't happen, at least not without an attack on some other systems as well. This protects against generic sort of "drive-by" attacks but if someone is being targeted then yeah, sandboxing the office suite isn't enough.