I sent the bitcoin from my account at an exchange. I did not use RBF or CPFP on any transaction. I didn’t even know what those terms meant, so I had to google them.
Someone did, perhaps the exchange. Although, I am not sure if exchanges do that. They usually overpay on the transactions they sent to avoid issues like long confirmation times. Mempool.space mentions CPFP in its records for one of your past transactions, which is perhaps what the casino meant when they mentioned doublespending to you. Ask the support which transaction was doublespent to see how they will respond.