That is also not substance dualism. Yes, what we see around us, including us, could be thoughts in the mind of God (idealism). There is no way to tell if idealism, materialism, or whatever -ism we might want to propose is the correct ontology.
Substance dualism proposes that there are two entirely different types of substance. For Descartes this was matter, which has extension in space, and thought, which has no extension. He couldn't arrive at a way for the two to interact, however. Spinoza argued that it was not possible for them to interact.