Irony refers to a contrast between appearance and reality. What does Schlosser mean when he says, "Calling any of these flavors 'natural' requires a flexible attitude toward the English language and a fair amount of irony"?

Flavors are literally artificial or synthetic; thus, to call them natural or real is an untruth since the two terms (artificial and natural) are antonyms or opposites. We can often find food products with descriptions or labels like "enhanced with natural flavors." This is irony in real life; although irony is often associated with and expected to only be applied in poetic or figurative language.