It's 51%, but actually, doublespending attacks on casinos are far easier to do.
You're right, but to explain a bit better... Basically, it only affects casinos that accept deposits on 0 confirmations, ie. as soon as the transaction is broadcast and detected by the casino's wallet.
1. Guy deposits Bitcoin with very low fee. Meanwhile he makes a second tx with the same bitcoin to himself, but with a higher fee.
2. Casino detects first tx and credits guy's balance.
3. Guy bets it all. If he wins. He withdraws. If he loses, he has his double spend to confirm with a higher fee.
It is weird people are diminishing the affects of double spending, multi accounting or anything else that hurts the casinos, hell even the 100 satoshi faucets in other websites are hurting those websites when people abuse it, why would you not really sympathize with those casinos just because they are not human, well they are run by human if that will make you feel better.
I get that people see a lot of gamblers losing money and they somehow justify that casinos do not "deserve" that money or something but in the end casinos are business as well and the owners are taking that hard earned money to their homes to pay for their bills as well, so there is no reason to wish ill on casinos and just be patient if you are stuck because you are mistaken for an abuser and if you are not then nothing will happen.