I grew up in the countryside and the article's conclusion of "let your kids eat some dirt" has always been rather anecdotally obvious to me; I tromped through a lot of mud and unwisely put a lot of weird things in my mouth and I seem to have a much more robust immune system than my peers who didn't.

That said, "growing up in the countryside" isn't itself necessary. You just need to grow up with some decent nature within walking distance. Yes, many urban environments have unwisely paved over all their green spaces with parking lots and high rises. But that's not a necessary feature of a city; look for cities that value green spaces, e.g. Pittsburgh's Frick Park/Schenley Park/Hazelwood Greenway triangle provides ample opportunity for getting lost in unmanicured woods.

Part of what gets my goat is the idea that humans only evolved to live in bucolic, pastoral locales. Humans have been banding together in cities for 6,000 years now; we have plenty of adaptations for urban living (such as, ironically, disease resistance; the notion that city living is cleaner than country living is a modern phenomenon).