Probably these people use a definition of theism that says that a god has to be an ontologically basic entity in an absolute sense, not just relative to our universe. If our simulators are complex entities that have evolved naturally in their physical universe (or are simulated in turn by a higher level) then they don't count as gods by this definition.
god has to be an ontologically basic entity
Also, the general definition of God includes omniscience and omnipotence, but a simulator-god may not be either, e.g. due to limited computing resources they couldn't simulate an arbitrarily large number of unique humans.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.