"Sinclair was considered a muckraker, or journalist who exposed corruption in government and business.[4] In 1904, Sinclair had spent seven weeks gathering information while working incognito in the meatpacking plants of the Chicago stockyards for the newspaper. "
Turns out it doesn't actually "expose the fiction of 'The Jungle.'" It cites "a 1906 report by the Bureau of Animal Industry" -- and nothing else. It then runs headlong into a classic libertarian diatribe against regulations.
And even if "The Jungle" /was/ pure fiction (instead of just fictionalize) -- readers didn't know that. And, despite their revulsion, the stockyards kept chugging along.