Early on in my blogging (and in the development of Leonardo), I was thinking about new ways of thinking about comments and I decided they were really just a form of annotation and had a relationship to trackbacks.
The way trackback worked (I don't know if they are still a thing, at least in the form they had 10 years ago) was you pinged a blog with information about a reference that had been made about an entry in that blog. The information could include an excerpt of what was said.
The trackback implementations I'd seen tended to give a trackback URI for an entry that ws different from the URI for the entry itself. A more RESTful approach (and one I planned to implement in Leonardo) would be to have the trackback URI be the URI of the entry. So you POST to the blog entry to trackback.
I had already considered POSTing to the blog entry as the mechanism for comments and that is when it first struck me that comments and trackbacks were really the same thing. Comments are just trackbacks without a reference to an external resource (sort of like self posts to reddit or Hacker News nowadays).
Because Leonardo treated a blog entry as just another page on the site, building this for Leonardo would have meant a fairly generic web page annotation system.
In my blog post on this ten years ago, I also talked about the notion of the annotations being out of band (i.e. the statement is stored in a third location, not the same as the subject or object).