But in most "threaed forum" examples, including several in the article, such as HN or Reddit, the post contents would be in the body of the DETAILS and the SUMM

But in most "threaed forum" examples, including several in the article, such as HN or Reddit, the post contents would be in the body of the DETAILS and the SUMMARY for when minimized is just the title or a summary of how many comments are below. This article isn't suggesting putting the whole post in the SUMMARY, though the given example doesn't likely make that clear because it isn't a full example.