Nudges are often imagined as just how choices are presented, but yes the default option is considered part of nudge theory. As also is social proof ("Your friends picked this choice").
also, the question is how much structural elements influence outcomes (not merely decisions), not whether they do or not. that’s the extra complexity of a social system built atop a biological system built atop a chemical environment built atop a physical one. we’re complicated. physics is nigh child’s play in comparison.
https://en.wikipedia.org/wiki/Nudge_theory#Types_of_nudges