A Runefang's S is not relevant, because it is always wound. In fact, there is much debate about what a Runefang's S "counts as" for the purposes of special items which rely on the S of an attack - and I don't think there is a clear answer.
But a Runefang is NOT a Great Weapon - specifically because the description does not say it is, and also because of evidence in the Marius Leitdorf entry (where he fights with a Runefang in one hand and a dagger in the other). It is also the case that the Runefang model in the general kit is a single handed weapon, not two handed (and there is a two-handed weapon in the kit).