SEO and content marketing are not the same thing, but they go hand in hand and overlap. Content marketing does not eliminate the need for SEO, nor can it overtake SEO, it is hard to separate the two. Think of them as being the yin and yang that form one circle while overlapping. It is correct to view content marketing as larger and broader, while SEO is perhaps more narrow and technical in nature. For SEO to be broader, it has to endeavor into content marketing and for content marketing to succeed it must implement SEO.
SEO can make demands and run the show, and content marketing will fulfill the demands. Therefore, they complement each other like two best friends! Successful SEO demands content and simply put, content marketing is content! There is no such thing as SEO without content or keywords. Applying keywords, researching them, putting them into a practical application is content marketing. You have to use keywords strategically through well written content.
Even building links boils down having high quality content making people/businesses wanting to link back to it which means high SEO success. To get a link back you need content that people want to link to. Search engines, especially Google like fresh content. Sites need a steady updated, output of content. Consistent output is content marketing whether you realize it or not. If you work on SEO you are working on content marketing and vice versa. They are not disconnected and both are essential for each other to operate at a successful level. Both will fail if they are not incorporated together.