How were the ten generations between Noah and Adam all upon faith, considering that Idris was sent to a polytheistic people before Noah?

The summary of the answer is that what has been narrated from the Salaf concerning the interpretation of the Qur’an and historical accounts does not have the same ruling as what is established by thean or the Sunnah. Furthermore, the mentioned period between Adam and Noah, the belief that the ten centuries were upon faith, the sending of Idris before Noah, and the emergence of polytheism during the time of Idris—all these matters are subjects of scholarly disagreement, and there is nothing definitively established regarding them in the Book or the Sunnah.