"A very unusual situation, though perhaps not alone in it. The Earth I know doesn't have anything like that, at least not any towns that have been recorded."

And isn't that the key part? They wouldn't be known of if they weren't recorded and no one would record them if they were hidden.

"Though magically hiding certain locations, warding them is a very common practice."

Brakebills itself was hidden to those that weren't to know of it, though it was a much smaller place. And all Library repositories were warded to blend in with their surroundings. Similar practices at least.