No one mentioned wealth but you. The "king of the north" might still be "ruralite" while a poor guttersnipe would be an "urbanite".
The only implication is that people living off the land, who need to cut lumber, haul water, till crops, and work to survive might, just might, be more physically fit on average than those in a city who rely on other skills.
First, a division by civilizational complexity is inherently a division by wealth. Relative wealth within that category may be different, but measured as an economic trend, complexity yields more extracted value.
In other words, even the Grand High Poohbah of Dirt Farmers is still selling his crops upriver for that guttersnipe to buy (or steal) FROM him, in the hopes that some of that centralized wealth the guttersnipe has access to (or can steal) comes back to him. Cities can't exist without rural populations, and rural populations are thus economically lower on the foodchain than anyone in a city (even a guttersnipe). The flow of capital is up the chain there.
Second, it's not true that country folks are necessarily more fit on average than city folks. It's a stereotype. The game doesn't need to support that stereotype.
As I said elsewhere, we can dump the "subrace" terms for humans and call it "origins" or "region" or "heritage".
....or "Background"....?
The intent is more to make humans more mechanically interesting, to make the stat boosts of the other races more appealing, and to make humans as diverse as all the flavour says they are.
Eventually WotC will want alternate humans in the game, be it from the shadow plane or the like. And it would be nice if there was an easy mechanical way to swap out human racial traits.
Sure. And when they do, perhaps subraces will be something the game can explore, when they are fully and obviously ensconced in make-believe magic land where people can create darkness because someone in their family once got herpes from a osyluth and where neandertals can walk among us as caveman stereotypes since that has nothing to do with actual people.
But lets steer away from the "Oh! The Laboring Class is naturally suited to doing Labor in the fields! And it is only right and just for those born to the Manager Class to direct them! It is their Natural Talent!" kind of implications. I get that fantasy is stepped in those implications already, and maybe individual games will be, but I don't think they need to be enshrined in the basic rules of the game.
Backgrounds? Sure. That's just character history.
Subraces? Nope. That's what your character
is. And humans -- especially in a fantasy setting -- are the ones who can be anything. They are not defined by what they are. They have the ability to shape their own fate. So it shouldn't matter where they're from. You can be born to a dyanstic legacy of rulers and administrators and still be a better ranch-hand than you are a lord. You can be born in a barn with the cows and you can still be a better king than you ever will be a farmer.