I think extra properties outside of physics conveys a stronger notion than what this view actually tries to explain. Property dualism, such as emergent materialism or epiphenomenalism, doesn't really think there are any extra properties other than the standard physical ones, it is just that when those physical properties are arranged and interact in a certain way they manifest what we experience as subjective experience and qualia and those phenomena aren't further reducible in an explanatory sense, even though they are reducible in the standard sense of being arrangements of atoms.

So, why is that therefore an incomplete understanding? I always thought of qualia as included within the same class of questions as, and let me quote Parfit here, "Why anything, why this?" We may never know why there is something rather than nothing in the deep sense, not just in the sense of Larry Krausse saying 'because of the relativistic quantum field', but in 'why the field in the first place', even if it is the only logical way for a universe to exist given a final TOE, but that does not hinder our ability to figure out how the universe works from a scientific perspective. I feel it is the same when discussing subjective experience and qualia. The universe is here, it evolves, matter interacts and phenomena emerge, and when that process ends up at neural systems, those systems (maybe just a certain subset of them) experience what we call subjectivity. From this subjective vantage point, we can use science to look back at that evolved process and see how the physical material is architected and understand its dynamics and create similar systems , but there may not be a deeper answer to why or what qualia is other than its correlated emergence from the physical instantiations and interactions. That is not anti-reductionist, and it is not anywhere near the same class of thought as substance dualism.