I have 2 speculations, not sure if one is correct? 1. The author submitted a link, then made a regular comment, and a mod decided to move that comment into the body. 2. The author filled both the url and the text fields when submitting the post, in which case the text would be hidden by default, and a mod decided to reveal it.
https://news.ycombinator.com/item?id=30509847
I just filled in both the url and text fields and it just worked for me.