This really applies to any rookie quarterback, but since Darnold is/was getting hyped somewhat based on his preseason performances I'll put it here:
Was listening to Dan Patrick Show this morning and they interviewed Joe Thomas. He said fans should basically take nothing from the preseason games. He mentioned how in Brady Quinn's rookie season Quinn started the fourth preseason game and he drove right down the field for a TD and everyone was super excited. Joe the pointed out that the opponent played basic Cover 2 defense the entire drive, which basically gives the short middle throws to the offense, and the defense never adjusted to take that throw away because it was preseason. Nothing really for the QB to adjust to, just look in the same general area of the field for the relatively easy completion.
Not saying Darnold or any of the rookie QBs will flame out like Brady Quinn, but just the efficiency/stats/performance/everything is pretty meaningless in preseason.
There are warning signs to look out for of course. For example, hospitality tents. ?
Edit: But the hyping and excitement and doomsdaying and arguing is why this is fun entertainment, so let's carry on.