I understand correctly that thanks to the blockchain, the lottery will be fair? It turns out, it is impossible to forge the results of the lottery.
No source codes are available, no data flow and execution diagrams too. So it is hard to say if lottery results can be manipulated or not. They claim that they are 100% decentralized but suspicions are that some centralized servers and involved. Product is not autonomous so some people and/or server(s) are needed to make it running ... and it can stop at any time if those (centralized) services will be not available.
Your argument about the non-public source code is very important. As long as they don't show it publicly it is normal to assume that it's not 100% decentralized and might be prone to manipulation.
Also, what is the point in hiding the source code? I'm kinda split on this one. Having an open/public source code (all of them) adds credibility to the project and it's kinda expected nowadays, but as a (somewhat) developer myself, I can understand the reasons for wanting to keep the code hidden. Especially now when projects are being forked or copied all the time, I wouldn't want to expose all of my work for someone to copy it and possibly create a competition for myself. But on the other hand, I can't have a total confidence that the project is 100% legit or decentralized or whatever if they keep a major portion of the code hidden.
The FireLotto team stated earlier in the thread that they will release the code once the ICO is over to protect the "patent" from competition, so we'll just have to see.