Yeah, that's what I remember, too. Man, it's hard to believe that was 15+ years ago now. I feel so old!
But yes, people were convinced that the KH2/FM secret endings were secret trailers for KH3, which wasn't too out there considering KH1/FM's secret endings were teasing KH2. I remember me and my friends were all convinced that the three armoured figures were Sora, Riku and Kairi until the Final Mix secret trailer came out.