This claim of some Jewish people that Uzair is son of Allah (SWT) , was common among Jewish communities in Yemen and Madina during the time of the Prophet Muhammad ( SAWA). There is Jewish sect called Asbahaniyyah which was also claiming the same. That is why, when the Prophet Muhammad (SAWA) recited the Quranic verse on them, none of Jewish people denied it, despite all their enmity against Quran and Islam. If it was not among them, it would have been a golden chance for them to attack their enemy, the Prophet Muhammad, but they admitted it because it was among them at that time.
The Jewish people kept on changing their texts all along the history till now. They have now removed that claim from most of their books which are available to the public, but that does not mean that it was not their at the time of the Prophet Muhammad (SAWA). Their admitting and not objecting or denying it is a very clear evidence that it was a fact.