According the EULA, it seems to me only the integration part need be distributed free and with source, but not the library itself. So in SkookumScript’s case, they only need to make SkookumScript UE4 Plugin open source and free of charge, but they can still provide core SkookumScript library in close source form. (Though I do believe all UE4 3rd party library should come with source, the whole close-source middleware thing is so last century in post-UE4 era)
If there’ll be C# integration in UE4, I would rather it would be the donet CORECLR backed by MS, even it’ll takes much more time to mature, mainly because it’s MIT licensed.