Not only is it open source, it's free software! I am very pleasantly surprised to see that the GNU GPLv3 license has been chosen. I've been harsh on Light Table because the source was nonfree and the promise to "open source" it eventually didn't look promising or community-friendly. I expected a permissive license or an open-core strategy to monetize proprietary components and not be friendly with the free software community, but that doesn't seem to be the case here. I look forward to seeing where development goes.
Happy hacking, Light Table devs.