Yeah, for some reason, HN disallows a top-level comment on a submitted link (but you can do a top-level comment on a stand-alone/no-link post). I always found that odd - as a submitter, I generally want to provide some commentary on what I found something interesting.
Worse, their submission form looks like you can provide top-text, and indeed that same form field is used as top-text for Show HN, but for a normal submission it puts that text in a normal comment which almost always reads weirdly in that context.
