My pet theory on this is that it's because most vulcans are vegan or vegetarian, and many humans still eat meat and other animal products. The diet thing is confirmed canon, but I can confirm being able to smell... meat smell(?) on people who eat it.
The meat Federation citizens eat isn't really an animal product. In the Orville, it's actually made explicit that killing an animal for food is regarded as tantamount to murder in the Union.
The reality is it wouldn't matter if it's synthetically produced meat, it's still going to have the same proteins and chemicals as it would if it came from an animal, which is what people (myself included) can smell on people who have consumed them. Eating garlic makes you smell like garlic, eating dairy makes you smell like dairy, eating meat makes you smell like meat.
I think the implication that we're smelling the concept of sin is very funny though.
it's not a theory, it's fact: the smell is rotting flesh (and dairy for most) and animals on earth rely on it to know if something is nearby them.
