Great idea! Considering the state of their app I don't see them having the time and resources to have tight integration within the app, but even just an automatic post for each video and a link in the description would be great.

I imagine not many creators are terribly keen on the task of moderating such a system. Even if the amount of assholes is reduced by having to pay for it, it may still be nonzero. I can also imagine it to be a liability issue.
Plus there's no need for it.
Nothing stops you from discussing their videos with the community on any platform. Those who have access will be able to use the link. The YouTube crowd can join in once it's up there, but most will probably stay on YouTube.

I think they said they see no need for comments on Nebula for the time being