How, in FANG alone there are THOUSANDS of HR members that rotate in and out like a revolving door. They barely know eachother let alone any of the workers save a few executives they worked for.
You took the statement too literally. Of course there is not a single whatsapp group, but hundreds of them. People working in HR have lots of friends who also work in HR, meet HR friends of their HR friends, they talk with each other, and so on.
Don't believe this practically