Assassins aren't the only AC that can cloak/sap/regen. And Merkave isn't the only Sin/Shadow in the tournament. Merk isn't even the most skilled Sin/Shadow on the server. Hell, I'm sure he counted the cool million he made and said "man, its a good thing C.K. didn't enter with his operative, that thing is OP."
Here's why he won. He looked at the rules and said "this gives me the best chance of winning, I want to win it all." If the rules were different and another class had an advantage... He probably would have played a different class. And probably still would have won.