I don’t think this was one of those cases. The Italians I know have firmly held beliefs about what foods should be eaten when and why that don’t seem at all grounded in science. If the bodybuilding subculture has developed beliefs about food, they’re likely just as firmly held.
The bell peppers in particular sound like believable pseudoscience. nightshades are poisonous and if one is looking to avoid bloating, perhaps it’s a legit concern.