Yes, because this was while the Prophet (saaws) was travelling along with his companions and not while he was in the mosque. The Prophet (saaws) went to perform ablution but he was late so the sahabi lead the prayer. When the Prophet (saaws) came to join the prayer and he saw that the prayer had already begun, abdur-rahman ibn awf was about to step back but the Prophet (saaws) gestured him to carry on leading.
He lead the prayer before the arrival of the Prophet because he did not want to lose the rewared of praying the prayer at it's earliest time. He did not lead the Prophet in his mosque because in the mosque only the imam can lead unless he apoints someone else. And the Prophet (saaws) did not repremand him for doing this.
This is from the top of my head and I will have to re-check my notes to confirm that I did not make any mistakes.
The Prophet Sallallahu Alayhe Wasallam then said, every Prophet was led in Salah, and the one who will lead me is Abd Ar Rahman ibn Awf. (or something to that effect.) This is on the top of my head too