That’s interesting. I didn’t think to check the HTML source and just assumed that it was JSON-LD.
Google uses schema data, but I’m not sure if they use that specific one. The schema.org docs aren’t written very clearly.
It looks like Discourse is putting multiple DiscussionForumPostings on each topic, but the example in the docs looks like DiscussionForumPosting might be meant to refer to just the main topic and not the comments? The docs list a comment field with a Comment (singular) though the description is worded in plural.
I just looked at how Invison does it and it uses JSON-LD, putting Comment objects in a comment field. It looks like it’s a lot of extra text to send to the browser.
I don’t know what the answer is, but I’ll try to research it more later.