There's no selection bias here (other than the obvious, blogs using disqus), since they are using aggregate data. They make the false claim that pseudonyms contribute more though which does not follow from their data. They should show comments per user per login source data to make such a conclusion. Their current data only hint that using a disqus account is easier than facebook or anonymous.

I don't think there's such "comment racism" in disqus. I 'd argue most disqus users barely notice they are using disqus.