I agree with you and will go one further: why would a coach put the rookie in this position with Jimmy G.? Sitting on a bench isn't in JImmy G.'s best interest. He is going to want to play as long as he can. Also, there's the optic that the coach and gm would not trust a QB they draft by hedging him with Jimmy G., thereby setting up a situation of a QB controversy should Jimmy G. not fail.
With Davis Mills, they already have a proven quantity. They can have the same effect with him. The only thing I can think of is that they are targeting AR and want him to sit for a year, so the Love in the FGB situation or everyone's favorite example in Mahomes. Either way, this makes no sense, which is perfectly in line with the Texans's SOP and the kind of kryptonite situation that Jimmy G should run away from.