The code has an internal analogue, and the tooling lets you choose whether to export the entire git history or squash it. They may have chosen the former, in which case it could just be 2 years to migrate and rework the code to be ready for open sourcing. In that time I imagine there were four reorgs and countless priority shifts :)
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.
Either they rewrite git history or it took about 2 years to get approval on making this repo public.