What is the interpretation of the Almighty’s saying: "And they testified against themselves that they were disbelievers," and why did He not say "polytheists," despite their acknowledgment of God’s existence, as evidenced by their saying: "And we have reached our term that You appointed for us"?

Disbelief (kufr) and polytheism (shirk (definition)) may sometimes be used interchangeably with the same meaning, and at other times they may differ. Disbelief (kufr) is the denial and rejection of truth, such as denying the obligation of prayer or the prohibition of adultery. As for polytheism (shirk), it is the devotion of some acts of worship to other than Allah, such as seeking help from the dead or sacrificing to them. A disbeliever is also referred to as a polytheist, and a polytheist as a disbeliever, as stated in the Holy Quran and the Prophet's Sunnah (definition). An example of this is abandoning prayer, which has been described as both disbelief and polytheism.