I don't think that's right. Simulations don't necessarily study or represent real phenomena. You most certainly can simulate things that don't exist.
Good grief, the entire genre of Science Fiction is predicated on the question of what happens when you add X where X is often something that not only doesn't exist, but often can't ever exist. Dune's Spice comes to mind here. Yet writing about a universe where we have a limited substance that allows for instantaneous travel, extends life and can do all sorts of other things is quite possible.
So, no, a simulation doesn't have to be based on anything real. You just need a system which sets initial parameters (which need not be reflected in reality) and then have that system tell you information about what happens once you set things in motion.
Granted, what's a "real phenomenon" does seem to change rather a lot over time.