That doesn't mean anything Low 1-C wise unless proven, it can refer to a number of things that are not Low 1-C like their space, time and dimension being different. We also know Hell is located below the universe, universes in DOOM are called dimensions too, so this can simply refer to that positioning, as in a reality that is bound "by boundaries of space, time, or dimension" is in the positioning the universe is, but Hell is below all that and so it isn't bound by it. Needless to say, this is a way of talking that exists in fiction likes this, "I'm not bound by [things]" can mean being outside of them to take effect or different and what those "things" are can be said in a redundant manner.