I would see this in action taking cubs and boy scouts camping.
We would some nights cook over an open fire and some of the kids are like' We are going to cook on that , what a mess'.
I was 'Well we did it this way for about 100 prior lifetimes and we got this far.'
Then go to cook eggs, and they would be ' Where is the can of spray oil for the pan'.
I am 'Well kids we just cooked bacon, toss the eggs mix into the same pan' .
Then after dinner wash up and the modern scout way is detergent bin, rinse bin, dilute bleach bin, and hang on the line to dry.
When I was a kid we would skip the last three steps and all survived.
Yes I believe a little bit of dirt in your diet, ( not just eating but life experience as a whole) particularly when you are young is important.
My wife is a City raised gal. When we started getting into outside activities she would get a mosquito bit and swell up like mad.
I asked - 'No back yard camp outs, etc where you saw mosquitoes.' She said 'No, never'
Now after a few years her body adjusted, and now she gets a little itchy spot when the get ya, just like me when I get bit.
So yes, a wide and varied experience leads to a better attuned immune system in my book.
But that does not make me an anti vaxer at all.
Some things are so far out of our acquired experience we need a like kick start to tune our immune system to get ready for the boxing ring, and that is exactly what a vaccine does.