[spankalee]

If you know you want to open source a project eventually, it's easier if you start it in the open source part of the internal repo with all the licensing and headers in place. Open sourcing existing code is harder because you need to review that it hasn't used something that can't be opened.

So probably they just started the project two years ago, had aspiration to open source, and finally just did now. Some teams might publish earlier, some like to wait until it's had enough internal usage to prove it out.