What land do you know has only one attraction? Even the Harry Potter land at Epic has the circus as a placeholder until they get their 2nd "E-ticket" built.
Piston Peak is not a ride, it's a land that the Rally Racers ride is on! I'm calling it a mini land. You can call it whatever you want, but if it's not a ride, then what is it? I just made an entire post about the semantics of this. It will not only have a second ride, but the same (or similar) barrel bridges and possibly other features from TSI will be used as a way to walk around a certain part of the land, or whatever word you would like to replace with land.
The problem here is that you're taking what TWDC has done before and trying to deduce 'rules' based on that.
But that's not how Disney parks work. When the original DL was built, guests didn't walk around deciding to call a certain area "Tomorrowland" or "Fantasy Land."
That came from Disney himself.
And so, if TWDC wants to change the rules, they can. E.g., building a new theme park based on World's Fairs. Or building a park that doesn't have a hub and spoke design. They can do as they please, even if regular guests or super-fans are confused, or don't like it, or it sets off their OCD, or would have done it a different way (oh hai! armchair imagineers!)
So if TWDC wants to build a fully walled-off land based on one IP (as with SWL:GE)... they can, and they did, despite that's not how Walt did it.
And if they want to create mini-lands they can.
And if they want to remove a ride from Frontierland and put a new ride in its place and consider it still as part of Frontierland, they can. (At no time did any Disney marketing call Piston Peak a "land" or "mini-land.")
Because they can.
An example of this is Disney Princesses. It hurts the brain to figure out what make a princess or even a non-princess (oh hai! Mulan!) part of the club. Or how a 3D animation from Pixar (oh hai! Merida) can be part of the club. The answer is simple: TWDC says it is. All based on marketing and nothing else.
Go ahead and contradict Disney's appellations in your head canon. Just be prepared to be corrected if you say it out loud.