If your order was for $5 and through this glitch you got $5 on top. Would you want to give the money back no matter what ? I think most people would take the extra $5 and move on. Now if you got $100k plus $100k, this is a different ball game. There is an individual threshold with being "honest" and the higher the amount, the more likely you will be honest imo.
It's true that the size of mistake plays a large role, and people know that often times when a mistake is small, the company might not even respond to them, so it's not worth the effort to notify them. But with smaller business, it can be really important to disclose such incidents, because these mistakes might be vulnerabilities that can cause serious damage to them. And in our case, I think all crypto casinos are still rather small, and since some of them have public bankrolls, it's better to not ignore such things if you encounter them.