What is the validity of the statement that the people of David and Solomon - peace be upon them - were disbelievers and polytheists despite their monotheism of God?

It is not possible to affirm or deny anything concerning the peoples of previous prophets except what has been mentioned in the Holy Quran and authentic Sunnah, because other books have been subjected to alteration. As for what has not been mentioned in them, we pause and say: Allah knows best. The Prophet, peace and blessings be upon him, said: "Do not believe the People of the Book, nor disbelieve them, but say: 'We believe in Allah and what has been revealed to us.'" The verses of the Holy Quran clearly indicate that the peoples of David and Solomon, peace be upon them, were believing monotheists. This is evident in the story of David with Goliath, in the description of the family of David as being grateful to Allah, in the gathering of jinn, humans, and birds for Solomon, and in Solomon's command to his soldiers to make Balqis and her people Muslims. All these are proofs of their faith. Likewise, Balqis's statement: "My Lord, indeed I have wronged myself, and I surrender with Solomon to Allah, Lord of the worlds" indicates her submission and faith.