This seems to align with the idea that as children we are better if we are exposed to pathogens, and the body learns to react, but not overreact to them. The current thinking appears to be that if we don't then the body over-reacts to certain things and we get 'allergies' to common substances.
From what I have read, being exposed to animals, e.g. dogs and cats, can improve the immune system in young children, and being excessively clean can reduce the exposure to these pathogens and therefore result in an immune system that does not respond optimally.
This needs to be done at a relatively young age. Once the body has learned how to react, it is too late, or difficult to change.
No, just to upset you, building your own forest and playing in it as an adult is not going to protect you from coronavirus. Its not the way it works, and this article is not discussing that.