If you look up the word κέρατο in ΧΛΝΕΓ, the closest idiom you'll find is
πιάνω τον ταύρο από τα κέρατα: αντιμετωπίζω με ταχύτητα και αποφασιστικότητα μια δύσκολη κατάσταση.
But I wouldn't say this has much to do with the usage you're citing. My interpretation is that the ambassador in question felt certain that he had such a firm grip around "truth", that anyone attempting to express views other than his own would be doomed to be regarded as an ignorant fool.
A (very) loosely related (and somewhat vulgar) idiom that some native readers may associate your citation with is the following (again, from ΧΛΝΕΓ):
πιάνω τον Πάπα απ' τ' αρχίδια/από τα γένια (προφ.): θεωρώ ότι είμαι σπουδαίος ή ότι πέτυχα κάτι σημαντικό: Νομίζει ότι έχει πιάσει ~ ~
