when physicists push back against simulation theory using the notion that it would take a universe worth of energy to simulate a universe, they are assuming the same physics are at work in the real and the simulation.

when we make up a game world, it has its own rules and restrictions, some emulated from our world, some unique to the simulation.

There is nothing stopping our reality from being a simulation by beings operating under entirely different rules of physics and simulating something on the scale of the universe is their version of No Mans Sky - a procedurally generated universe defined by algorithms and not hand crafted.

and within no mans sky, players have recreated basic games. give them more tools and they can create more complex games within the game - simulations within the simulation.