What you are referring to is a "slug" [1] which adds another searchable dimension to the content. This appeals to marketers trying to add searchable keywords to boost discovery. Not having one won't affect HN since "everything's present" in the forum. For instance, if you google the title of your post it is ranked #1 in the search results [2]

[1] https://en.wikipedia.org/wiki/Semantic_URL#Slug

[2] https://www.google.com/search?q=Ask+HN%3A+SEO+impact+of+HN%2...