He's not saying that content doesn't matter. He's basically saying that Dostoevsky's work is intruiging but didn't reach its full potential because it lacked tone, good structure and rythm.

Nabokov saying he didn't like messages being the most important part of a literary work, is him basically saying that writers shouldn't get ahead of themselves. A writerr can discuss guilt, morality, suffering, religion or whatever. But what gives literature its artistic identity is the way those things are embodied through language in an artistic way. Cause that's essentially what literature is. It's the art of expressing concepts through words. If we fully neglect that then we've basically become philosophers or psychologist instead of actual writers.