Derivational etymology, correct in its principle. Ἦτρον is indeed a thematic derivative of ἦτορ. This was correctly perceived by some Greek grammarians, on whose work Eustathius relies. However, the formal derivation path is incorrect. Since ἦτορ was assumed to be derived from "to blow" (see ἦτορ / ἄημι), the etymon of the etymon can be presented as the etymon of the lemma, resulting in "ἦτρον is derived from ἄημι"—which is what Eustathius says in the second part. The derivation chain is ἄημι → ἦτορ → ἦτρον, which is abbreviated into ἄημι → ἦτρον (see ἦτρον / ἄημι)