1. GitHub is footing the bill — I'm paying for gitignore.io (although it's only costing me the annual domain)
2. The site uses 3 Dynos — Currently gitignore.io uses 1 Dyno on the free tier and I've recently moved the backend from Node to Swift to double / triple network performance based on my preliminary testing. I don't know why the site needs 3 Dynos because like the OP mentioned, it's a static site. I also use Cloudflare as a CDN which could dramatically improve git-scm's caching layer. It's not that helpful for me as most of my requests are dynamically created, but for a static site, it would drastically reduce Dyno traffic.
3. Access to Heroku seems to be an issue — I ran into the same problem and I'm finishing up a full continuous integration process to build and test my site on Travis. I basically want to approve a pull request and have the site fully tested though my Heroku pipeline, then have the PR landed in production.
4. Traffic - I don't know how many users he's got but I'm seeing about 60,000 MAU's and about 750,000 requests a month.
* Jason Long helped design my site and logo as well.