Possibly it's making a subtle equivocation between "earth" and "land", that is, the Dutch obtained a lot of what is now the Netherlands by extracting underwater land from the sea (or used to, something like that). It's not just saying that the Dutch "created their nation" in the sense of laws and whatnot, but actually "made" the land for it.
My guess, anyway.
That's the interpretation given in this French children's book, where I first encountered the proverb.
Here's the new thread for posting quotes, with the usual rules: