They are actively working against open source tooling for software development by developing features only for their closed platform.
Gitlab is a much better example of a company that fully embraces open source.
GitLab Enterprise Edition does not have an open source license. Since GitLab.com runs GitLab EE, their two (presumably) primary sources of revenue (EE licenses and GitLab.com paid accounts) come from non-open source software.
But: it's still fair to call GitLab as a company way more Open Source than GitHub. All of their development happens out in the open, the vast majority of their codebase is Open Source, and the source code for GitLab EE is even made available.
This is a spectrum, not black and white.
To me in order to be a Open Source company your primary product must itself be Open Source.
RedHat as an example... RHEL is open source
GitLab.. GitLab is open source
These are open source companies
Hiring a few devs for work on some side projects that are open source does not make one an Open Source Company
If they open GitHub Core then they can claim to be a open source company
Also as mentioned elsewhere in this thread, Github has published plenty of open source software. Electron, Atom, resque, and updates to git itself.
As I clearly stated, GitHub can not be a Open Source company simply because they hired some devs to work on Open Source
Do you consider Microsoft to be a Open Source Company?
Which is a Microsoft product, and Microsoft is the inventor of EEE. Ergo, GitHub is complicit in an EEE attempt. Q.E.D. Case closed.
/s
I'm sure they employ direct maintainers/contributors to other open source projects as well.