[waves at @tomchick]
True, and that argument should have force for Luther given his doctrine of sola scriptura. Catholicism doesn’t have to treat all words in the Bible the same way, but interprets them in light of tradition. The notion that communion involves the real presence of Jesus started very early in Christian worship, could reasonably have come from the apostles, is bolstered by the later Gospel of John (“unless you eat my body and drink my blood”) and by Paul, and was passed on through many centuries until the Reformation.
(Tagging @legowarrior in case he has anything to add/refute)