This seems like a strange argument. Suppose that the creators created some simulation. This argument would apply equally no matter which simulation parameters they choose.

Is there more here I'm not getting?

When they do simulate one, wouldn't the entities in that simulated environment make that exact same argument, and be wrong? If there are now two universes, that one and the real one, you've got a 50:50 chance. And I think we can rely on super-universes to simulate "interesting ones". Look at Greg Egan's works. Somewhere in the Super Universe, there is a Super Greg Egan saying "Imagine if quantum computers didn't work. What would that look like? Wow, they wouldn't be able to solve this kind of problem: in my novels, the protagonists will have to invent some kind of computation, and then they will run into this kind of problem and call it NP-hard. Poor bastards." And then some kid will read Super Egan's book and simulate, and here you are arguing it can't happen, because "random".

You don't have enough information to conclude anything about the simulator's universe just because they thought of this one. Consider wakamoleguy's argument about Pacman above: Pacman could apply that same logic to conclude our world is like his, but the simulation's rules just don't impose such strong constraints on the simulator.