The author of the article has every right to know what some company is doing with the private information of the customer, but this doesn't mean he is entitled

The author of the article has every right to know what some company is doing with the private information of the customer, but this doesn't mean he is entitled to being explained the (technical) inner workings of the application. If Whatsapp was a bad citizen in respect to battery life I would at least understand why he is asking, but this is not the case. What I am sure of: Whatsapp maps E.164 normalized telephone numbers from your contact list to every contact in the phones address book. The app probably queries you phones contact database just like the standard phone or SMS app would do for every bit of information it shows.