Yes, at least on all *nix systems I have ever seen, root can become any other user with no need to enter a password, just
su. I don't see how that means that "the creator's uid has higher privilege than root" though.
That article is simply saying that some of the android daemon's commands cannot be run by
root. That doesn't mean that someone else has higher privileges, only that the command will test and if it is being called by
root it will refuse to execute.
So, yes root can
su to any other user but if it does, it is no longer root so that's not relevant. The point is that some commands should not be run by users with elevated permissions (like root) because they are dangerous. If they are run by someone who has logged in as root and then switched to a normal user, they are being run by a normal user, they have no way of knowing that the user in question happens to have the root password. As long as the UID of the user who called them is not 1, they're happy.