I happened to be on a scholastic portal when I saw this question. That, and that alone, is the reason I took the time to find the following. I swear I have a life! ;) But, I'll share what I've learned.
I could not find any evidence of the phrase existing before Korzybski, and the following is NOT proof in any direction. But it is interesting.
Lewis Carroll wrote a piece, penned in 1889 and published by Macmillan and Co., called Sylvie and Bruno and Sylvie and Bruno Concluded which uses remarkably similar language and reflects (I believe) the same concept.
You can see the Google Books version here.
We very soon got to six yards to the mile. Then we tried a hundred yards to the mile. And then came the grandest idea of all! We actually made a map of the country, on the scale of a mile to the mile!
"Have you used it much?" I enquired.
"It has never been spread out, yet," said Mein Herr : " the farmers objected: they said it would cover the whole country, and shut out the sunlight ! So we now use the country it-self, as its own map, and I assure you it does
nearly as well.
The concept almost certainly predates any modern philosopher, but finding an example of that specific phrase would be a tough undertaking.