I absolutely disagree on that one. A QB doesn't have to play bad, that he gets a bad rating. There can be good pressure by the front 7 so he doesn't have the time to complete his passes. There can be great coverage on the defense backs, so the QB doesn't find his open receivers. There can be great interceptions play on otherwise good passes by a QB. Starting every drive in your own 10 yards line because of great punts by the opposing team, etc. So in my opinion there can be many different reasons why a passer rating is that low.
I'm definitely not saying Siemian played a good game, but I can't agree on saying that a QB has to have a bad game for having a low rating and being exclusively responsible for a loss. There is a saying over here in Switzerland that we use in Soccer: "You only play as good as your opponent lets you to". So in my opinion the Dolphins played a good game in all three phases of the game against a bad Broncos team. But we didn't let them to have a better game so the win was deserved.