30 Philosophers, Chapter 25, Spinoza, Touchstone 65: Monism.
Monism says reality is ultimately composed of a single substance. For Spinoza, that substance was the one infinite reality he called God or Nature: everything that exists is part of it. TST takes a more limited position. We know of one shared substance, the Material World science investigates, but that does not prove other substances or realms cannot exist. Monism asks a deeper metaphysical question: is ultimate reality made of one substance, two, or many?
The TST Split itself is epistemic, not a declaration that only the Material World can exist. The Material World is the one shared reality available for empirical investigation. You are free to believe in additional realms such as Heaven, Hell, Purgatory, or Valhalla, and you are free to explore the number of substances. TST simply asks us to recognize that such ideas are not established in the same way as the Material World we all share. In this sense, TST is monistic about the reality we can establish, but makes no metaphysical claim beyond that.