They are basing this off it's monthly earnings from April but personally I can see it's monthly earnings increasing further. The holidays should be insane and maybe even a knew map announced at e3.
It won't be easy but this game has the potential to make 5BN in a 12 month period. It may be free but don't let that give you the illusion this game isn't making bank. And with the likes of Sony taking 30% cuts in digital revenue the platform owners are also making a pretty penny off of this.