I believe the intent is that the gardener and the winnower are mostly metaphorical, but represent some sort of primordial beings that represent the light and the dark in its purest form, to the point that they’re more of a concept than an actual being. Similar to Gaia in Greek mythology, or maybe more comparably the idea of “Mother Earth.” I think the traveler and the witness are more similar to angels serving their masters, the light and the dark, or even similar to us, the guardians, serving them.