SEO is a technical practice, content writing is not.
A SEO professional has to have knowledge of markup languages HTML, CSS, java maybe PHP and what they can do to optimise a website is performance (navigation, UX etc).
SEO is analytical, content writing is not.
The role is essentially analytical of the website under their control and other websites linking to them. They must keep up with daily search engine changes and general chatter.
SEO used third party applications to increase traffic / user experience beyond on-page content
It demands behind the scenes work like optimising user experience with robots.txt, enhancing metadata (canonisation), schema markup, and a sitemap (technical SEO). That means content found on the website is what the business wants to be found. Non-relevant pages are hidden.
So where do SEO and content meet. Websites must have content. There is no SEO without content.