Publish on Packagist
Looks like the Human Made folks have forked this repository in order to make it easier for them to use it with Composer. For this, they have published it on Packagist:
They don't want to rely on a third-party repository like WordPress Packagist to get their dependencies. Plus, since that site only mirrors the WordPress.org repository, you won't get pre-release version through it.
To make lives of developers and agencies using Composer easier, we could publish the plugin on Packagist ourselves.
Doing so is pretty straightforward, and requires no code change whatsoever. In fact, all that's needed is hitting a button on https://packagist.org/packages/submit. Since we already use GitHub releases to share the built ZIP files, Composer will automatically pick these up when downloading packages. So people will always get the proper version.