The word used for matter by the article (not peer reviewed) and equated with the absolute subject is "ὕλη", hyle, matter. But "it is unclear what has to be taken as Aristotelian matter", so likewise its never being a predicate.
- The traditional view is that matter precedes substance, and it is the substratum of substantial change only.
- Others claim that, in both substantial and accidental change, matter is a substratum of sensible substance (it just persist or doesn't).
According to wikipedia, what cannot be a predicate of other things, for Aristotle, is the Hypokeimenon.
Matter is especially and primarily the hypokeimenon which is
susceptible for generation and corruption [when nothing accidental remains of the other], but in a way also the
hypokeimenon for the other changes, insofar all hypokeimena are
susceptible for contraries. (320a2-5) [ibid]
So in the second view, matter is the hypokeimenon in all change, because it is their substratum.
In the former view, matter is the hypokeimenon even when not the substratum: it is a "third thing" that accounts for e.g. the persistence of a body that becomes unwell.