I can believe it back then, when the big mac was either on the dollar menu or very close to value prices. I wouldn't believe it in 2024 where the sandwich alone is $5 + tax (disclaimer: high CoL area).
Big Mac is probably the worst burger because of that stupid middle bun, but it's not awful by itself to eat. 700 calories or so. It's the drink and soda that double that and really puts you over.
Wasn't there a guy claiming to eat only Big Macs in the documentary?