One, per the "experts" (PFF and others), it appears Rush threw two interception worthy balls, not 4-5.
Two, you are missing the larger point here. Rush is a backup. Hence why Dak was starting and not Rush. Dak gets paid oodles of money to be an elite QB. Rush gets paid to be a backup. The range of what we accept as "good" from Rush is wider than Dak's because we expect less from Rush than Dak.
Three, as you said Rush is there not to "lose" the game. Because that's his ceiling. Dak is paid (and considered by you and others "elite") to "win" games. There is quite a difference here.