Maybe it's a workaround for the issue where HN hugs itself to death if comments get over a certain number.
My understanding is it wasn't because HN depends on S3 but because everyone was on HN at the same time talking about it!
Edit: my google/HN Search fu is failing to find a reference, but they can't easily horizontally scale HN because it is running on a single arc process. Comments/Submissions are stored in text files, and the "More" links are actually stored in memory in closures. When the closures get garbage collected you get the "Unknown or expired link." error.
It was the volume of comments in a single thread.