Undun is a character sketch. There's not much of a traditional narrative. 'Arc' implies change- introduction, buildup, climax, denouement, etc. You could argue that undun has all of that, but it's so subtle that it's barely discernible. It's more like a bunch of vignettes about Redford that don't necessarily connect in terms of a narrative.
Maybe the articles I read about undun wanted it to be something that it really isn't (a story). Again, it's more of a portrait.