I'm guessing a sloppy equivalence due to the idea of "existence is outside of the world and outside of space and time" showing up in descriptions (e.g. "a platonist might assert that the number pi exists outside of space and time" from https://iep.utm.edu/mathplat/).
To be more specific: if the Universe (existence itself) can be described by mathematics, and mathematics is timeless (beyond mere physical existence), then essentially the physical Universe is inevitable and in some sense "created by" mathematics. In this view of creation, mathematics plays the role of God.
