You are right, of course, but the point is to sell stupid books to gullible people.
terra meant earthly, as in terrain, which wouldn’t make much sense in the context that was intended in the phrase “novus ordo saeculorum.” The word ‘saeculorum’ was used in describing a length of time attached to a worldly process, a word encompassing both time and space in relation to a specific existence. Saeculorum in use is a word that has always been much closer to the word “world” in breadth and scope than the word “age” which is a more limited in reference to time. In fact, saeculum is the earlier root of the word saecularis, from whence we get the modern word secular, which means worldly (in a neutral sense), as opposed to heavenly.
And in response to the conspiracy theorists:
Yes, indeed, America was meant to be the New World Order, and we would spread the torch of liberty across the globe, not by force or coercion, but by awareness of self-evident truths. The founding fathers were by definition illuminists, and they lived during a time what we refer to as the age of enlightenment. These were all good things and necessary for the further promotion of mankind.