You seem to make that statement with authority. What if time loops? Then you can travel far enough into the future to arrive in the past. Making broad statements like, "So and so said it can't be done means it's impossible." is simply not a very bright direction...still others with IQs likely higher than yours or mine once said that flying machines are impossible...or the act of travelling faster than sound in the atmosphere.
I figure after we or something more or less akin to us becomes living gods we can pull of just about anything....and that may be ions before we even arrive at that stage.