We also get their rejected malt. The megas have tight tolerances for malt. Anything that falls outside of those tolerances gets rejected. Rejected malt finds its way into less demanding distribution channels.
Speaking of Briess, Briess 2-Row is one of the most inconsistent malts in the homebrew trade. Some bags make great beer. Other bags produce beer that is not so great. I seriously doubt that AB Inbev accepts the latter type of malt from Briess when they purchase 2-Row.