Thank you for this. The author may have simply made a mistake. That's quite possible. It's also possible that he was referring to wheat beyond the producer's own need -- exports -- and if this is the case, it appears that Russia and Ukraine produce somewhere in the neighborhood of 30% of the combined total of the top ten producers -- see here https://en.wikipedia.org/wiki/List_of_countries_by_wheat_exp...

So, not quite a third, but not much of an exaggeration.