Yea, this stuff needs some clarification >.>
Honestly its kinda scary wording because its all forked onto my personal (public) repo but holds the same license... legal gray area heh.
That being said, I think there is plenty of room for "improving" the Spine runtimes simply by adding auxiliary support scripts. Like the SkeletonRagdoll stuff I just added, I don't think it was necessary to include it in part of the runtimes, it could've been hosted as a unitypackage here on the forums instead and avoided all the licensing stuff. I think most people's improvements to Spine fall under that category. I guess the biggest question is what is the proper way to 'contribute' that stuff.