I asked them for proof of this "opposite betting collusion" which I'm sure they don't have because other than playing normally in the casino I didn't do anything else.
Opposite betting usually related to sportsbook. It is possible in some casino games, but the house edge always helps the casino to get profit in such cases. So, it is clear that your issue is related to sportsbook and opposite betting shouldn't be unknown to you. Based on your post history, you aren't new to the sports betting. Were you placing bets on both outcomes?