What proves that the two verses Zayd ibn Thabit found with Khuzaymah al-Ansari are two verses from the Noble Quran, and what negates the possibility of other verses existing with other Companions?

Abu Bakr and Uthman's selection of Zayd ibn Thabit for the compilation of the Qur'an was due to his presence at the final review (al-Ardah al-Akhirah) of the Qur'an, and because he was a scribe of the revelation. It does not preclude entrusting a person with a task even if there might be someone more suitable, as long as he is capable of performing it. Some of the mentioned Companions were not present in Medina at the time of Abu Bakr's compilation of the Qur'an. Zayd's authentication of two verses based on the testimony of a single Companion does not mean that other Companions had not memorized them; rather, it means that these verses were not found written down except with that Companion. The Qur'an is preserved by Allah's preservation and His promise, and the existence of verses with a single Companion does not prove their Qur'anic status unless they are mass-transmitted (mutawatir).