Well, presidents may not be THE power behind the scenes....but they aren't stupid either.
They wouldn't have made it as far as they did if they were, or couldn't be trusted to tow-the-line.
Neither Trump, nor Harris, nor the previous presidents and nominees BEFORE them were stupid enough to believe they could get into the Whitehouse and "wing it" and just go buck wild doing whatever the hell they WANTED to do with no accountability to their "handlers" who actually put them in those positions.
They may have SOME power to do a few things, but the policies are pretty much set and their job is to represent those policies with a smile on their face.
The fate of John F. Kennedy (and a couple others before him) serve as an example as to what could happen to them if they decided to stop dancing with the man who BROUGHT them to the party.