@belloq87 says it how I see it. He can't just wish his way across timelines. He needs the machine. And everything they show in the movie, it has to be the same machine. Tony's math doesn't account for alternate realities and timelines. Steve is not Tony. Even knowing he was in another timeline doesn't mean he can change which one he's in.
As for the Russo's. If you had a plot hole in your multi-billion dollar movie, would you say it's a plot hole or just say, no, you just don't know how it works. wink wink nudge nudge?