That seems to be so, at least it seems to be where the officers thought it was. His left arm was visibly pinned by at least one of the officers, and you can see his hand out and empty. But his right arm was what the officer on top of him struggled with for several seconds, and it seems like that after the 5-second mark, the officer on top also had his own gun drawn in his right hand.
Apparently around the 4-second mark is when one officer screamed about him having a gun, and at the ten-second mark the officer shot. So a lot happened in a very short period of time.
One factor that seems quite likely is that the officer on top quite reasonably drew his own gun, and with Sterling apparently having a gun at his right side, which the officer on top seems to struggle with him for and to try to dig out from along his side, that officer was close-range to get shot if Sterling were to get even partial control of his gun. And the officer was struggling with him while having his own gun in his hand. The other officer drew his gun, but stayed clear of the struggle and just held his gun on Sterling while staying on his arm.