τοὺς μέντοι Ἕλληνας τοὺς ἐν τῇ Ἀσίᾳ οἰκοῦντας οὐδέν πω σαφὲς λέγεται εἰ ἕπονται
Is this an anacoluthon? One could use Gen, I suppose: τῶν μέντοι Ἑλλήνων τῶν ἐν τῇ Ἀσίᾳ οἰκούντων …
Yes, I think it’s an anacoluthon. In the preceding sentence, the construction is indirect speech, introduced by impersonal λέγουσι and consequently the names of the leaders and the specifications of the troops are in the accusative. So X. starts the sentence in question putting the noun phrase τοὺ̀ς μέντοι Ἕλληνας τοὺς ἐν τῇ Ἀσίᾳ οἰκοῦντας in the accusative, as if the verb is again going to be λέγουσι, but then shifts to λέγεται, leaving accusative noun phrase dangling. Maybe an editor might insert a dash after οἰκοῦντας to highlight the anacoluthon.