I think it's more complicated than dirt.
I think diet can be very influential for people with acne. Besides, if food can rot your teeth, contribute to heart disease, create problems with irritable bowel syndrome, and damage your liver, why is it unbelievable that it can affect the largest organ system in the body (skin)?
Basically, being healthy, including how you eat, is better for your skin than the alternative. The less processed food and alcohol I consume, the less acne I have.