If what they're claiming is true... then it would seem that we have culpability on the part of Roobet for continuing to operate a "broken" system... but also culpability on the part of Yahoo for knowingly using said "broken" system. 
Scam? no... total clusterfuck? yes
Scam? no... total clusterfuck? yes
Intent matters though. If yahoo purposefully attempted to place max bets knowing the casino would not pay out, then it's an issue. There isn't evidence of this. However, this isn't a scam accusation more than it is a reputation issue. It's not okay that Roobet is allowing users to place bets that exceed the max win because they are gaining the benefit of a large bet loss without the downside of having to potentially pay out that large bet. Good that this thread was moved to reputation. It's a bit deceptive but it's not clear that Roobet did this with malice considering no other players faced this issue. It's reasonable to give them the benefit of the doubt here but it's grossly irresponsible.
IMO what's most fair:
A settlement payment of some sort would be best because there is negligence in both parties. Paying out the full 9k is excessive but somewhere along the lines of x0.3 - x0.5 of the win amount would be appropriate.