They mitigate the risk by settings caps to maximum bets. You can't just bet a bit more than half of a casino's assets in double or nothing with them. They don't want to be taking a 50% chance of losing everything. Even if there was a 2% edge that's still a 48% chance of it happening. They would rather work with smaller amounts where the chance of someone winning a ton of times in a row to eventually win everything is practically 0%.
>The stablecoin on the other hand claims that stakers are being rewarded for risking their capital without holders of the stable coin being fleeced at their expense.
They may be rewarded but they aren't being guaranteed that the value of what they hold will always go up.
Again a Ponzi scheme is a very specific thing. People make an investement and returns on that investment come from new investors. In this kind of stablecoin system there is no part that where that specific thing happens.
>were supposed to enjoy average negative returns for the sheer joy of gambling like people on craps tables
This is usually how algostables work. People bet on growth of the project and once growth has peaked negative returns will be coming. By trying to avoid this period of negative returns a depeg happens.