Hopefully by "Open Source" they're meaning something OSI compliant, and not abusing the term as some others have done.
I found OpenToonz GitHub project page[1]. There is no code yet, but it seems that they will publish as an open source software.
I hope they also prepare English documents.
Any download links yet, or are they still putting together the free version?
The toonz website says: Effective Saturday March 26, the TOONZ Studio Ghibli Version will be made available to the animation community as a free download. - so a few days yet. (site is http://www.toonzpremium.com/, is linked in the cartoonbrew article.)