Have you actually read it yourself beyond the title? It doesn't permit the US government from granting anyone titles of nobility. It doesn't prevent anyone from holding or claiming such a title on other grounds.
I suspect that you're confusing it with the Titles of Nobility amendment, which went further by stripping citizenship from anyone who would accept a title from a foreign country (so even under it self-claimed titles wouldn't count) - but that amendment was never ratified and is not standing law. Some people claim that it "has actually been ratified", and hence is part of the Constitution "that the government doesn't want you to know about" - usually this is claimed by fringe right-wingers, the type of guys to the right of the Tea Party.