The mystery is "why this gesture" and not "are they communicating"
The hypothesis that's being put forward is this:
Apes, they argue, do not inherit specific gestures, but they do inherit the sense that they can use gestures to communicate with other chimpanzees. The animals create new gestures by borrowing — or “recruiting,” in the scientists’ lingo — movements that apes commonly make.
They'd probably get less engagement if I did that; headlines that confuse people cause them to comment, which in turn causes Facebook and Reddit to show content to more people.