The "ἀγροῦ πλέως, i.e. boorish" ("full of rusticity") refers only to ἐξ ἀγροῦ λέγεις, not to the negation (οὐδὲν). The Oxford boys are often confusing like that.
Cf. Hesychius <ἀγροῦ πλέως>· ἀγροικίας πλήρης "full of boorishness"
Cf. Michael Apostolius Paroemiographus, Collectio paroemiarum Centuria 13, section 43, line 1 <Οὐδὲν ἐξ ἀγροῦ λέγεις:> ἤγουν οὐδὲν ἄγροικον "i.e. nothing rustic/boorish"