They clarify it here: https://www.opera.com/blogs/news/2016/04/opera-doubling-serv...
"Our VPN feature is still in development. We are currently working hard to implement support for proxying even more of the browser traffic, including WebRTC and plug-ins. Having this functionality built into the browser, instead of as an extension, allows us to catch more situations, such as certificate revocation checks made by the system. Yes, the VPN feature is free, and we do not plan to charge for it. Our VPN is something we call a browser VPN. Under the hood it works by routing all the browser traffic properly encrypted via our secure proxies in various parts of the world. It will not route the traffic from other applications – as a system wide VPN would do – it’s a browser VPN after all."