[misja111]

This has nothing to do with scaling. How do you picture that, some employee would broadcast sensitive information to a WhatsApp group, in order to reach as many others as possible at once?

The offense here is that no effort was done to keep records of the communication. It would have been ok to use WhatsApp if they somehow would have archived all communications. Records of communications have to be kept so that auditors can verify that no inside trading secrets were communicated to others, for instance.