It is unclear whether this etymology, absent from the main manuscript of Orion, is a true etymology or a semantic justification which was mistaken for an etymology by the copyist, who inserted ἤ (see a parallel in ἄνηθον / ἄνω + θέω). Assuming it is an etymology, the word is parsed as a compound of ὕλη, which goes back to the fact that ὕλη can mean "firewood" and is as such a synonym of ξύλον. The initial [ks] is assumed to be the adjective "dry", which was pronounced [ksiros] in Byzantine Greek, so that the [I] could be "shared" with ὕλη pronounced [ili]. If originally it was only an explanation, it explains the λεαίνεσθαι, the word can be polished because it is dry.