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?