You are not wrong. But he is not wrong either. He is not saying that OP has a 67% of getting the mount. He is saying that between all players getting for a 5% drop mount this event, on average 67% of the players will get the mount in 20 tries or less. The OP is on the unlucky 33% percentil of players.
You are in the unlucky 0.5% percentile. It means 99,5% of players will get the mount in less attemps than you. It sucks but per every 1000 players it is expected on average to have 5 wich such bad luck.