Considering all of the suits that were at the end, they all had to have a separate power source to work and be controlled by Jarvis. All of them could not be powered by Stark. Since the Mk. 42 could be remotely controlled, there's definitely no way that would have been powered by him. I can see where the confusion lies, especially considering the first 2 movies, but I can't take that much issue with that plot point. Tech evolves. Or something...