How is it that Sentinel Prime is featured in the movie if it was declared in Revenge of the Fallen that Optimus was the last Prime?

Though it is not explained yet, there is a theory that The Fallen could not detect Sentinel's presence as he was in stasis lock aboard The Ark, or knew that Sentinel Prime was on the Ark, and posed no threat. It was the Fallen who (arrogantly) stated that Optimus was the last Prime and as such, the other Decepticons naturally believed (or at least, did not question) him.