WhatsApp sets a lot of store by the privacy in messaging it supposedly offers. However, allegations that links that invite new users to private groups are searchable online have recently emerged. This is due to the fact Google has indexed them, meaning appropriate terms can bring them up.
https://www.notebookcheck.net/App-researchers-report-that-WhatsApp-group-invites-are-searchable-online.454505.0.html
Totally a manufactured problem. Facebook can't stop people from sharing invite links to their groups publicly. Journalist just wants a story :P