It looks like dang has a page despite having no slug on his profile page: https://dang.at.hn/ https://news.ycombinator.com/user?id=dang Unless of course he added it, and later removed it?
i think the dev of at.hn just added him for testing reasons.
Surprised to see one of these kinds of sites be opt-in. Most things I see using HN data are real loosey goosey about it.