It always grabs the last part of the domain. Its just a quick app hosted on heroku, so all foo.heroku.com stuff will show up on here as heroku.com. Happens with google.com, etc too.
On the front page right now there's instagram-engineering.tumblr.com, erratasec.blogspot.com and tech.blog.cueup.com. It seems to be a based on a domain whitelist. Heroku should definitely be on it.
Personally I'd just show the whole hostname and maybe prune a few common subdomains like "www.", "www2." or "blog."
Definitely, I would be surprised if this post would get to the top otherwise. A vague title with the heroku domain has probably attracted a lot of clicks.