“Human is a species” was a natural proposition in the earlier distinction, because its predicate concerns a nature itself. “Human is universal” now illustrates a mental proposition, because universality pertains to a concept considered in the intellect.
Both concern something other than humans taken one by one as individuals, but the questions differ.
Natural: Is the judgment about the nature itself or its instances?
Mental: Is the judgment made concerning the concept as mental rather than external?
The two labels should not be treated as automatically synonymous.