The excerpt that provides story details that would likely be absent if Buck were the narrator is:
“Dave refused to run quietly on the trail behind the sled, where the going was easy, but continued to flounder alongside in the soft snow, where the going was most difficult, till exhausted.”
This detail focuses on the actions and choices of another dog, Dave, and his experience, something Buck would not fully understand or convey as he would be limited to his own perspective and experiences.