It is not necessarily a mistranslation, as one can chalk it up to the ambiguity of the word in question. It is true that the word 'corn' is often understood to mean the plant also known under the indigenous term 'maize' (Zea mays) but it can also be used as a synonym of 'grain'. The latter meaning is much older, as it existed in Old English, Old Norse and Old German and is probably derived from Proto-Germanic kurnam meaning 'seed'. The shift in meaning came in 16th century, when the maize has been nicknamed 'Indian corn', what eventually has been casually shortened to 'corn'.

Nevertheless, the word consequently used in the original text is 'frumenti' which is a counterpart of the English word 'grain', without specification of the particular species of the original plant (wheat, barley, millet etc.). In other words, there is no mystery here - original Latin text uses a perfectly ordinary Latin word that has been somewhat ambiguously, although still correctly translated by Thomas Holmes and also by e.g. William A. Macdevitt.