Op if you don't play at betterbets then there is a motive behind your post. I'm going to guess extortion probably. This looks like some kind of oversight error. They fix it in minutes....now we can pick client seeds in the 4 billions on accounts. So dev said they spoke to you and it seems outcome wasn't what you wanted -gimme money or I'm posting- and you made this post. Does this about sum it up?
They fixed it mainly because he contacted them and asked about this issue. Without him we probably wouldn't be aware that exploit like this can be even possible.
I don't know if he demanded money for this, but if I were the owner of BetterBets.io I would honor Op with some kind of small bounty for his bug hunting.
Same thing I would probably do in other to save the reputation which is even more important than the bounty even if he asked for one. The issue is the fact that he noticed it means someone else might noticed it and instead of raising it as OP has done, the next is scam accusations but I must say I am glad the way the issue was resolved instead for allowing it drag for too long to handle...