Thank you for your question. It is not agreed upon that he did lead a congregational prayer on behalf of the Prophet (saw) the days before his (saw) death. In fact there are instances where Abu Bakr was about to lead prayer and the Prophet (saw) despite his illness and the weak state of his (saw) body got up to lead the prayer.
For the sake of argument that he did lead a prayer, that was not put forward by Abu Bakr himself as a reason to be the successor of the Prophet (saw). The event of saqifa shows that those ghat believe in the validity of the Caliphate of Abu Bakr do so on the basis that the Prophet (saw) did appoint a successor.
May you always be successful