You're right, perhaps I should have been even more assertive that "not running up the score" isn't that big a thing even in American sports.
I think a clear case of this though is when a team has the ball with the lead and less than 2 minutes to go, and the other team has no timeouts, it's customary to just take a knee ("victory formation") rather than going on a drive and trying to score again.
That's not about not running up the score though. That's about doing the absolutely safest thing to run out the clock. Teams will do that even if they have a small lead because it's almost foolproof. (I'm sure some quarterback has fumbled though.)
I think a clear case of this though is when a team has the ball with the lead and less than 2 minutes to go, and the other team has no timeouts, it's customary to just take a knee ("victory formation") rather than going on a drive and trying to score again.