I get the impression from some of the BoS that the Deep is sort of a place (as well as a force/entity/thing). Also Xol’s line in The Whisper: “you shall drown in the deep” (or something to that effect). Perhaps IT is too metaphysically vast for lesser beings to comprehend, so instead we interpret it as a void, the same way the back of your hand may seem like a desert to a dust mite.
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<The Deep in that case probably references the core of the Ascendant Plane, IT's realm.>