"Our founders and engineers have a strong open source background and we consider a contributory relationship with the FOSS community as the normal course of business. Thus, our plan all along has been to make our entire client-side code base open source; however, as anyone who has worked with such issues knows, it is often not quite that simple."
So they say they want to be good, but not quite yet. I've posted a question about nimbus.io on that page.
It might be more appropriate if the site had text like "if you are interested in the source code, please email us." As it stands, it does seem like they are selectively releasing the source (and this is an assumption, right, since the site doesn't say "request an invite and get the source?").
It's open source behind an invite wall. Once you get it, it should come with the OSS lincenses. I'm assuming.