What makes you think the Army have anything to do with Whatsapp? The article says the leaked document is being sent to other people using Whatsapp. That's equivalent to saying it's being shared using email - it has nothing to do with how it was originally accessed or how it was leaked. Indeed, it's rather odd to see El Reg using phrases like "available to download on Whatsapp", given that there is no such thing as being available to download on Whatsapp. If someone sends you a message with an attachment, you can view that attachment, just as you could if someone emailed it to you. You can't go wandering around Whatsapp looking for things to download.
The actual news here is that the army appear to have really shitty privacy procedures involving confidential documents being available to tens of thousands of people with essentially no security or access control, making leaks inevitable. The details of which messaging services have been used by various people to share a document after it has been leaked is of no interest whatsoever.