This statement is hyperbolic. You can only be "in the middle of somewhere":
In the middle of Paris; in the middle of New York; in the middle of a class room; etc.
The expression "the middle of nowhere" is used to claim that you are far away from civilization. The desert is considered "the middle of nowhere" or the jungle or somewhere equally empty of people.
This expression uses the word "nowhere" as exactly the opposite of what the word means. The word "nowhere" means a place that does not exist. But the expression says that you are in the middle of "somewhere" called "nowhere".
To make the expression more confusing, the entire term "middle of nowhere" is now used as the new "somewhere". And now, you find yourself in the middle of that new "somewhere":
In the middle of somewhere called - "in the middle of somewhere called -'nowhere' ".
Finally, the writer tries to make a literal point when he states the fact that you can not find the center to "nowhere".
So, in short, this statement makes no sense other than saying that they were in a very remote area.