I'm familiar with Iocaine Powder. The fact that it won an internet Roshambo competition doesn't mean that it's the dominant strategy. If the opponent knows what move Iocaine Powder is going to play, the structure of Roshambo tells us that there's always a move that beats it. If it is not performing well, then it falls back to completely random. Completely random is a perfectly reasonable thing if you're in a Roshambo contest, not so much if you're trading money because transaction fees will eat your profits.
Yep. Kind of like paper-rock-scissors, there is no dominant strategy.