This. 100%.
The Relationship Story is often presented as “the argument between the MC and IC” but I think that is better handled with the dynamic between the Main Character Throughline and the Influence Character Throughline. To then put “their argument” in the relationship is redundant.
I’ve found massive improvements in quality of storytelling when the author focuses on the relationship as if it is its own entity - the relationship suffers problems because of a Situation, not because one person or another has a problem with the Situation. When you start thinking of one to the exclusion of the other you start to collapse the potential of that Throughline.
A Situation Relationship Throughline often deals with classism – upper class vs lower class.
Many of the examples were written 20 years ago and trend toward a more simplistic understanding of the Relationship Throughline. Yes, it can be “an argument between two people” but again–you’re just repeating what is in other Throughlines and you’re missing out on a key part of your story that will elevate the quality of it 1000x.