Some of the oldest web forums (circa '98) I've been to use the nested comment format you see on Reddit or on here. The flat timeline format because much more popular from what I've seen.
Reddit style forums are not great for posts intended to log something in progress (like a computer build) because they require the user to make a new thread multiple times instead of making a new comment that automatically brings the thread back to the first page. But flat list threads are great for that.
They usually try to compromise with comment replies shown in "comment pyramids" but they can get unwieldy quick. It's a reasonable tradeoff for the flat chronological layout.
And this is the only top level reply. Interesting.