Kuhzayma: The Only Witness
Everything about the history according to the Hadith regarding how 9:128-129 was included in the Quran is highly suspicious. But for the sake of understanding their narrative, let's look at the history based on their claims.
The Hadith claims that no one compiled the Quran into a single book until after the prophet's death. This task started during Abu Bakr's reign after the Battle of Yamama in December 632. During the battle, a large number (~70) of the Quran memorizers (qurra) were killed. Because of this, Abu Bakr and Umar were concerned that the Quran may be lost, so they commissioned Zaid bin Thabit to lead the effort and compile all the verses into a single book.
Before accepting any verse, he was to have at least two witnesses.
Narrated Abdullah, he said: Abu Tahir narrated to us, saying: Ibn Wahb informed us and Ibn Abi al-Zinad informed me, from Hisham ibn Urwah, from his father, he said: When the killing of the Qur'an reciters became widespread on that day, Abu Bakr feared that the Qur'an might be lost. So he said to Umar ibn al-Khattab and Zaid ibn Thabit, "Sit at the door of the mosque, and if anyone comes to you with two witnesses about anything from the Book of Allah, write it down." حدثنا عبد الله قال: حدثنا أبو الطاهر قال: أخبرنا ابن وهب وأخبرني ابن أبي الزناد، عن هشام بن عروة، عن أبيه قال: لما استحر القتل بالقراء يومئذ فرق أبو بكر على القرآن أن يضيع فقال لعمر بن الخطاب ولزيد بن ثابت اقعدوا على باب المسجد فمن جاءكما بشاهدين على شيء من كتاب الله فاكتباه. Kitab Al Masahif Ibn Abu Dawud p. 12
According to their narrations, he accomplished this for every verse except for verses 128 and 129 of Sura 9, Bara'a, where it was only with one witness, Khuzaima ibn Thabit al-Ansari.