HN appears to encourage it, because it shows a text box which becomes a top level comment when you submit, although it isn't obvious that will happen.

Huh, that definitely explains it. I wonder how many people know that. In that case it's particularly unfortunate to downvote the OP simply for filling a field in the submission form! Sigh… I guess it's another case of LLMs having made the world a little worse for everybody.

I don’t think it’s because the comment was submitted. I think it’s because it reads like LLM output.

Personally, I’ve only ever provided a summary if I felt the headline wasn’t clear enough.

Yeah, but before LLMs we didn't have "reads like LLM output" as a downvote reason. In 2022 nobody would've had qualms with the phrasing of the comment.

That box doesn't become a comment, does it? It goes into the description that shows under the link.