Because Oda has been inconsistent with introducing new elements into the story, and the wider world perception of it. Something like Haki, just everyone in the world knew about going forward, even though this kind of makes zero sense story wise. Without an explicit confirmation, I just assumed that everyone but Goofy and his friends knew about the Gods Knights.
Though I don´t think it really matters tbh. I´ve never been so hung up on the statement, the WG and the world at large including people in the know treat the admirals like the strongest fighting force anyway. Only Harald is really an exception, and I´ve even sort of slightly downscaled his base version in retrospect