Body builders built up that metabolic rate, it's not something you can just construct over a few weeks in the gym. Their bodies are actively consuming a massive amount of calories while they're resting. What's more, they aren't just consuming anything and everything, their diets are very specific and focus on Macros. Reddit science and actual nutrition aren't the same. Healthy and long term weight loss/gain will always be more than just calories in/out, hence why The Biggest Loser is constantly being sued for the long term damages they've caused to contestants