In my opinion, the first chapter is given this title because it follows Buck as he goes from civilization to a life that is much more primitive than what he is used to.
At the start of the book, Buck is a beloved pet dog. He is strong and big, but he does not really need to do anything with that. He is completely pampered.
But then he gets kidnapped and sent to Alaska. Along the way, he meets the man in the red sweater. Now, for the first time in his life, he's getting beaten -- much more savage and primitive life, right?
At the end of the chapter, he's in Alaska and is going to have to start adapting to primitive life.
When the book begins, the reader is introduced to Buck. He has a wonderful life with a very loving family. He is well taken care of, and his life in general is quite "cushy." Some readers might even claim that Buck's life before he was dog-napped was a pampered life.
But Buck was neither house-dog nor kennel-dog. The whole realm was his. He plunged into the swimming tank or went hunting with the Judge’s sons; he escorted Mollie and Alice, the Judge’s daughters, on long twilight or early morning rambles; on wintry nights he lay at the Judge’s feet before the roaring library fire; he carried the Judge’s grandsons on his back, or rolled them in the grass, and guarded their footsteps through wild adventures down to the fountain in the stable yard, and even beyond, where the paddocks were, and the berry patches. Among the terriers he stalked imperiously, and Toots and Ysabel he utterly ignored, for he was king,—king over all creeping, crawling, flying things of Judge Miller’s place, humans included.
That paragraph makes it clear that Buck wasn't a lazy pampered dog though. On the contrary, he was quite active on his California estate. In addition to being active, the paragraph makes it clear that Buck's owners loved him, and they treated him like a member of the family. Buck, in turn, treated them like they were his family, because Buck truly believed that he was the ruler over everything.
All that changes for Buck, when he is taken from the estate, sold, and brought to the Alaskan wilderness. The area that Buck is now living in is much more primitive in general. There is simply less civilization, so the title makes sense in that regard.
But I think the main reason that the chapter is called "Into the Primitive" is because of Buck's experience with the man in the red sweater. Buck is firmly beaten and humiliated by the man, but learned a valuable lesson. He learned that might makes right in this strange new area. That rule becomes a law to Buck. He calls it the "primitive law," and throughout the book Buck learns how that rule affects him, other dogs, and his owners.
He was beaten (he knew that); but he was not broken. He saw, once for all, that he stood no chance against a man with a club. He had learned the lesson, and in all his after life he never forgot it. That club was a revelation. It was his introduction to the reign of primitive law, and he met the introduction halfway.