The leaked Jagex conversation can easily be verified by Jagex employees. Those conversation were quite intimate and wasn't meant to be leaked because it would be a PR nightmare.
I've censored the picture. The uncensored one can be found in the video. There they talk more about us in a not so nice light.

In the first picture you can also see who made the picture. They are using WhatsApp. White speech bubble = other, green speech bubble = user. Someone adresses the user by his name.

They can easily verify if the screenshot is real and if so they know exactly the culprit and that he does not work with the team.