At the bottom of this page of the website they talk a little bit about who they are and where they got this info. Here is some of that:

The majority of the economic and demographic data used in the making of the forecasts is widely available by institutions such as the CIA, IMF, UN, USG, etc. You can see the most relevant data at every single country's page. There is a tiny part of data coming from a variety of shadow sources such as Internet gurus, unsigned reports and others. But all these sources are from the internet and are of public domain for at least a minority.

It is already said that this website is non-profit, built on spare time and we provide our information and services AS IS without further explanations and/or guarantees. We are not linked to any government in any way, shape or form. We are not a death or satanic cult or arms dealers as some BS is floating around the internet on this topic. Take into account that the forecast is nothing more than a model whether flawed or correct. It is not God's word or a magic device that allows to foresee the future.