Instagram was way before Onavo acquisition. WhatsApp was officially announced as an acquisition 4 months after Onavo purchase. The article does say insiders say they used Onavo for WhatsApp purchase decision.

However I’m assuming you don’t do a $19B acquisition (and 10% of your market cap) in a month. Wouldn’t Facebook have already been close to or starting the process of trying to buy WhatsApp before the end of 2013? Onavo was announced as a purchase in mid Oct 2013.