I think the key thing here is that Durov has the money from selling stock in his previous company (12% w/valuation of 3-4 billion) which means he can afford to pay to support the app in the meantime without any external financial help. I do think there are different ways to monetize messaging outside of pushing advertisements into people's face or selling information and third-party paid apps are a good option.
In that regard I think the title is misleading: Durov plans to make money through the app eventually, just not through selling people's information or push advertising.