That's a completely illogical conclusion you're making. Just because Stanzi completed a certain percentage of his passes, or made so many tds per interceptions does not mean in any way that he would continue to do so at the same ratio if he threw more passes.
More passes means more exposure. Since Iowa was primarily a running team last year, Stanzi threw less. That meant opponents spent more time worrying about Greene and less time worrying about Stanzi, focusing their efforts on stopping the run, not stopping the pass. On the other hand, Iowa State was primarily a passing team last year. Opponents prepared for Arnaud and expected to see him throw the ball a lot in each game. These factors likely played into each QB's stats.
It doesn't necessarily mean that he wouldn't keep that ratio either. For all we know, if Stanzi were to throw as much per game as Arnaud, he might even get better after more game time passing experience. I doubt it, but that is possible.
In the end, the only facts we know are how many yards, touchdowns, intercpetions, etc. that Stanzi and Arnaud were actually able to accumulate over the season. And based on the actual facts, i.e. the recorded events that took place on the football field last year, Arnaud performed better than Stanzi.