If the number of extended discussions is uncorrelated with the post's karma (except maybe for strongly downvoted posts), and the number of extended discussion comments dominates the number of total comments, then that is evidence that correlations between the number of total comments and the post's karma are spurious.

If the number of extended discussions is uncorrelated with the post's karma, then they would simply add a random noise component to the graph. I think it’s pretty obvious from the graph that the signal to noise ratio is quite high.