What's the theoretical basis of this though? I don't have a theoretical background in computability, but in physics and there the complexity of a system very mu

What's the theoretical basis of this though? I don't have a theoretical background in computability, but in physics and there the complexity of a system very much correlates with the number of degrees of freedom. When simulating something, you want a bijection of the state of the simulation to the state of the system you are simulating, so you need to match (or exceed) the number of degrees of freedom. So if you know how many parameters you need to simulate another system until the simulation works, you can estimate how many internal degrees of freedom it has (degrees of freedom that are relevant to whatever you are measuring of course).