The etymology you linked actually says something different: > From a Vulgar Latin *lo, *illu. Masculine pronoun from Latin illum, singular masculine accusative

The etymology you linked actually says something different: > From a Vulgar Latin *lo, *illu. Masculine pronoun from Latin illum, singular masculine accusative of ille. Neuter article and pronoun form from Latin illud, neuter of ille. https://en.wiktionary.org/wiki/lo#Spanish https://en.wiktionary.org/wiki/lo#Spanish That means some uses of "lo" are derived from masculine ille while others are indeed from neuter illud. To answer your phonological uncertainty, the entry shows derivation from illud > *illu > lo.