As you say, there's no debate that DPC places Quinn as a SG. Yes, Quinn is undoubtedly the most prominent SG. As something of an antagonist, her role across the story is more significant than any other SG. That makes her special among the SGs, fully deserving of extra attention in the 0.8 others end scene.
I could buy the argument that Quinn has an end screen by dint of her role as an antagonist only if what was being highlighted in that scene was...her role as an antagonist. But it isn't. It's a romantic scene, and promotes her role as a romantic interest.
Or it just means players new & old struggle with the idea of their MC getting close to characters they ultimately can't 'get' in the end. DPC wants you to care about all of his characters, that doesn't mean they're all destined to be romantic interests. Some people pine after Riona or Jade too. Fling vs love.
As I said, the features of Quinn's path have more in common with the five MGs (hangout etc), which are what give some people the understandable impression that the relationship with Quinn is a deeper one than the other SGs. This cannot be explained away by simply saying Quinn is an antagonist, because again, what is being emphasised in those scenes isn't antagonistic, it's romantic. You seem to be suggesting that the romantic scenes with Quinn are
only there as a means to make us care about her as a complex villain. I think that's a major misreading.
Conversely I suggest DPC doesn't use the term LI, he only refers to MGs for those who have romance paths. So I think DPC has made it pretty clear none of the SGs are LIs. Hopefully we'll get some indication of where the Zoey thing is going soon at least.
He refers to MGs, but at no point has he he explained what he means by that; he certainly hasn't said they're the only ones who have romance paths. In fact much of what he's done in the game suggests the opposite is true. There is a debate to be had about the depth of those relationships, but increasingly it really isn't one that is characterised by the MG/SG distinction.