I think there's confusion as to what '3rd Party' is referring. Just my interpretation, but I think 3rd means a commercial venture, not a community modder. ie, I feel the passage isn't aimed at the community, it's aimed at potential 'business partners'.
As for terrain creation, I think the passage is referring to the creation of new terrain areas (outside the Black Shark 'world) for integration to make a larger DCS world, not modding of the area (textures, objects) that will be released with Black Shark. And from the sounds of it, ED will have a major level of control over it. Which is fair enough, I'm not sure I want any old 3rd party making shoddy terrain areas.
As far as modding goes, it would be pretty hard for ED to enforce royalties on, say, a new model for the A-10, or a BMP-2, created by a community member.