I also thought about both possibilities. Growing up in that time period we played outside daily at school and at home. In fact our parents wanted us to go outside. We usually didn’t go in until dark even in the Winter months so our exposure to the elements was much more than today. We also had no airconditioning so all of the windows were constantly opened even in Summer. We had a window fan to circulate the air. BTW, anyone remember when the only air conditioned building was the movie theater? Sooo, our exposure to germs was constant. That may account for it.
In MY case, it didn’t help at all. I was the world’s biggest tomboy, spending most of my day in the dirt somewhere if I weren’t in school. I encouraged both of my sons to get dirty, and breastfed both for over a year. I nicknamed my oldest ‘dirt monster’, he used to have permadirt in the creases of his hands! We all have allergies… :(