They did claim that they would open source it completely when all the backer rewards were fulfilled (see post 7 in this thread): FYI:Source code of Snapmaker Firmware
Which actually has happened according to the SM team: https://www.kickstarter.com/projects/snapmaker/snapmaker-20-modular-3-in-1-3d-printers/posts/2967921
So hopefully we’ll soon see it as a repository on their github page soon: https://github.com/Snapmaker