The part of the thought experiment that is deceptive is that, the simpler you make the 'rule following' sound, the more unbelievable (intuitively) it'll be that the room has any 'understanding', whatever that means to you. If instead you say that the person inside the room has to follow billions or trillions of instructions that are dynamic and depend on the state of the system and what happened before etc (i.e. modelling the physics going on in our brain by hand, or modelling something like our modern LLMs by hand), then it's not as intuitively obvious that understanding isn't happening.