But there is such a line. You can unplug a simulation. You cannot unplug a reality. You can slow down a simulation. If it uses time reversible physics, you can run it in reverse. You can convert the whole thing into an equivalent Giant Lookup Table. You can do none of these things to a reality. Not from the inside.

I'm not sure that the 'line' between simulation and reality is always well-defined. Whenever you have a system whose behaviour is usefully predicted and explained by a set of laws L other than the laws of physics, you can describe this state of affairs as a simulation of a universe whose laws of physics are L. This leaves a whole bunch of questions open: Whether an agent deliberately set up the 'simulation' or whether it came about naturally, how accurate the simulation is, whether and how the laws L can be violated without violating the laws of physics, whether and how an agent is able to violate the laws of L in a controlled way etc.