There's no way that missile barrage was enough to kill every other Autobot before drifting off. And the Ark was pretty intact upon crashing on the moon. Whats more weird is that they were all in the same position they were in when they left Cybertron. They ran out of energon? No, because again they were still in that battlestations position. If it were energon problems how come they didn't go into stasis-lock in a pod somewhere? The only other explanation is that Sentinel killed them all while they were drifting and then locked himself away until someone--Decepticons--came to rescue him.