I do realize that they are different genres, but I would say they are closely related. I think of stuff like Explosions in the Sky when I think of post-rock. The point I was trying to make is that this album would have still made sense without Choi's vocals because of how smooth and enjoyable the music itself was. "Indie" is such a vague genre that different people consider different things as such. What would you classify this as?
pop-rock. I like the point you're making about the strength of the instrumentals, and I agree that indie is a woefully vague genre, but that and post-rock are still nowhere close to each other. Most post-rock bands are on indie labels. But most indie bands are not post-rock. It's still a fairly small genre.