After reading this, it seems to me that "could" or "possible" simply means this -- I have used an algorithm for simplified simulation, and it simulated X. The discussion about what makes X possible is simply a discussion about the algorithm used, and a degree or method of simplification.

"Tomorrow I can do X, or I can do non-X." = I can run a simplified simulation of myself doing X; I can run a simplified simulation of myself doing non-X; and neither of these simulations produce an error. (Why? Because I am not simulating myself down to a neuron / atomic level. A more detailed simulation could show that with given state of my neurons, I will tomorrow decide to do X, instead of non-X.)

"The billionth digit of pi could be zero." = I can visualize myself calculating pi to billion decimal places. But my visualization does not include the critical details, which I simply replace with a reference class "digits in pi" or simply "digits". At the end I have multiple "possible values of billionth digit of pi" because the reference class I used contains multiple values.

In other words, "possible" means: using my imperfect information, these worldstates were not falsified.