What are the proofs that the interpretive text (such as Tafsir al-Sa'di) conforms to the Quranic text, and that the interpretive discourse is the true meaning of the Qur’an, even though the Qur’an was revealed in both wording and meaning?

Understanding the Arabic language is essential for comprehending the Qur’an, as language is a means of expression, understanding, and instruction; hence, every messenger was sent with the tongue of his people. Comprehending the Qur’an relies first on understanding the Arabic language, and there are verses that require knowledge of their occasion of revelation or what has been narrated concerning their meaning in the Sunnah or the sayings of the Companions. Ibn Abbas mentioned that the interpretation of the Qur’an is of four types: an interpretation known to scholars, an interpretation that Arabs know from their language, an interpretation concerning which no one is excused for being ignorant, and an interpretation whose true meaning none knows but Allah. The Arabic language is very important in understanding the Qur’an; it is one of the greatest sources of interpretation and is used to clarify the correctness of an interpretation and the error of one who deviates from the Arabic language. Nevertheless, language is not the sole source of interpretation, for the occasion of revelation takes precedence over the linguistic meaning when there is a conflict, as does the legal meaning. The correct understanding of the meanings of the Qur’an requires understanding the Arabic language and knowing the Sunnah and the sayings of the Companions.