This is terrible advice. The choice of platform should be made based on the value proposition. In the case of whatsapp, the business is based on undercutting

This is terrible advice. The choice of platform should be made based on the value proposition. In the case of whatsapp, the business is based on undercutting the price-gouging of SMS with a superior alternative for $1 per year. This makes it affordable and appealing to everyone, including (especially) J2ME and Android users in the developing word. If your app fits this profile, definitely go Android first. But.. if your app is more appealing to people with discretionary income, you are shooting yourself in the foot if you don't start with iOS because that's where the users with money are.