The quality of a squad is not the only thing that is important for success. The management of a team is more important than it actually. But we know that their manager is Guardiola and his system has been marvelous so far. Maybe they still haven't won the CL title. But they have been doing great in the EPL. And I agree with you on that they will still become the champions this season despite losing against Tottenham. It was already a tough match and not a surprising result. Tottenham have been beating them here for 3 games in the end.
With several new players joining City, it is only a normal resulys, that City still need a process to win games and made more solid team. this defeat will give guardiola a very good thing to fix the squad and formation in order to achieve the best results. It's still one game, not reflecting failure to the end. After a few games, i believe guardiola will find his best formation, and from there City will start to rise.