Smyth in 2954a
Hdt. has οὐκ ὦν (sometimes written οὔκων) to emphasize an idea opposed to what goes before (non tamen). Thus, ταῦτα λέγοντες τοὺς Κροτωνιήτᾱς οὐκ ὦν ἔπειθον by these words they did not however persuade the men of Croton 3. 137.
Smyth notes this is to be distinguished from either οὔκουν or οὐκοῦν and it is usually written
as two words in Attic, οὐκ (μὴ) οὖν which I assume in Ionic is οὐκ (μὴ) ὦν. Note that the
gen. pl. relative pron. is aspirated, so there couldn't have been attraction here.
Following Smyth's translation of 3.137, your translation is spot on.