I can't say from exact experience with CsVs, but with grooming experience with many double coated dogs (including my own husky) I'd say brushing a dog like this, that is primarily kept indoors, IS necessary. The problem is, dogs/wolves outside get soaking wet in the rain and often go for a nice swim here & there. They also rub up against things like trees and bushes. This all helps get the undercoat out. With house dogs, they don't have that same experience, especially since most owners would prefer if their dogs NOT get soaked in the rain.
This is why brushing is necessary, if the dog isn't getting wet to help the undercoat push out, it stays on the dog and builds up, which can get worse over time. Like I said, no personal CsV experience, but it's been the same with all the various double coated breeds I've worked with!