Except it's not, because you still want it to be associated with the site. If you just want to comment on something, you already have Twitter, Mastodon, Facebook, your own blog, and probably a dozen other outlets. What you're asking for is the added legitimacy of the site itself, without their consent.
It's literally like having a browser add-on enable comments on a website by appending the Reddit/HN thread after an article [1]. But with the added benefit of the user being able to choose from a number of algorithmic/community moderation strategies to apply to the existing comments in order to show/hide/rank them.
In an ideal comment system I believe that articles, comments and moderation events should come from three different, decentralized streams (like Atom) that the end user can subscribe to individually and that are joined at the end users client.
What he is asking for is the exact opposite of "the added legitimacy of the site itself". He's asking for a user interface to integrate content that does not come from the site itself.
That would be a lot cooler than another comment moderation system, of which there are already multiple open-source implementations. Could someone at least provide an argument of why Mozilla Talk is better than the existing solutions?