This has now been changed on the site to reflect "you must submit the original Certificate of Naturalization,"
You do not need to obtain a Certified True Copy if the U.S. government asks for a copy of your certificate for official U.S. government business. You may use a normal photocopy of your certificate for legitimate U.S. government business. For example, when you apply for a U.S. passport, you must submit the original Certificate of Naturalization, and you should also submit a normal photocopy that has not been authenticated.