Ok it’s just pre-season. But as a backup quarterback, most likely this is your season. Your chance to show what you got. Sure there’s practice, and that counts in terms of showing knowledge of the offense and ability to make throws in that setting, but it doesn’t show how you play in game situations.
And the closest thing a backup quarterback has to that is preseason games.
As a former starter with many games of experience in the regular season, you’d expect a guy like Trevor Siemian to show more command. More ability. He should be a little above the second stringers he’s playing against.