Owyn

No, the promise was that he became the "greatest hero in the world". It is NEVER specified that he ever becomes the #1 hero of Japan, only that Deku is working towards that goal. The hero rankings fluctuate and change based on present performances, with All Might only managing to stay in that spot so long because he was practically unbeatable. Deku wasn't even a full hero when he defeated AFO as he was still in high school, meaning that he would likely have been ineligible to place on the ranking list at the time anyway.

You seem to be confusing "WORLD'S greatest hero" with "Japan's Number One Hero". Never did Deku ever say he became Number One, only that he became a world-renowned hero and that he did. He saved the world by defeating All For One. The ranking system in Japan is based on who is consistently contributing most to society in the present moment, not who's contributed the most across their entire career. It's going to fluctuate.