You have a dilemma for sure. How are you to know what is true? Which side is telling you the truth? Well...
Professional writers can generally be spotted easily by the two or three style guides, the huge dictionary, the huge thesaurus, etc. Pocket protectors and black plastic eye glasses are a bonus. Really, good writing can be evaluated.
SEOs in general are not technical people and only parrot what they have been told and have read online. Short of a search engineer, who can really tell you how a search engine works? Well, you are in luck! Search engineers have a trait in common with those research scientists that created the technologies used in search decades ago. They like to discuss and write about technology. Many search engineers have written research papers and books adding to the ontology on the subject.
So this is where I would start.
I like to push back and see if my B.S. meter goes off. You can question both sides and see what you get.
Search is not about keywords or matching keywords. Googles original research paper states:
Automated search engines that rely on keyword matching usually return
too many low quality matches.
Search engines, since the introduction of Google, have moved toward using semantic analysis beginning in 1997, then moving more quickly in 2003, 2005, and then again in 2008 through to present day. What was learned in Googles Scholar, has been rolled into the search engine and has grown to staggering complexity. While each technical element is simple to understand, the complexity in the implementation is unfathomable. To simply say that good SEO is about a handful of keywords is completely wrong. Is it like looking at a handful of spots without understanding that you are looking at a leopard or a hyena. Silly.
Be that as it may, search engines evaluate content using linguistic semantic analysis. Remember your writers? Do you think they understand linguistics best? You better believe the writers should! Do your SEOs? Not likely. Linguistic semantic analysis allows a piece of content to be understood. I can have two pieces of content with the same few keywords that have completely different meanings. If search was about keywords, then how could I match a user to the proper page? I can't.
So where is your marketing department?
You must know who your audience is, know how and what they search for, what their concerns are, what problems they need solved, questions they need answered, what motivates them, how to engage them, and so on. Where is the market analysis, the persona analysis, the plan to address search user problems, concerns, questions, etc.? If you do not have a marketing plan, then your SEOs should be answering these questions. Can they?
What the writers should be receiving is a topic, the topic scope, how the topic fits into the plan, how the post is to overlap another topic, what it should address in the way of solutions, concerns, questions, etc. Your writers should know precisely how a work fits into the plan and how to measure if the work is complete. Think of each piece of content as having a business case tied to it. You cannot simply arbitrarily begin writing stuff. What a waste of capitol! Each post should have measurable goals and not just rank for keywords. Content must complement each other and not detract. What is the topic strength of each work and how does it fit the plan?
SEO is about establishing topic strength, linguistics, proper use of punctuation to establish meaning, completeness, flow, form, etc. SEO is about writing precisely and not about keywords. For this, both the SEOs and the writers HAVE TO work together.
With SEO, there is no standard. It is more a matter of whether they understand what is needed and can drive relevant traffic. This is done with extremely tight discipline which requires absolute and complete understanding how content is evaluated.
I would be asking to see a plan. Any plan. Where is the market analysis, the persona analysis, the plan to address search user problems and concerns, etc. Ask your SEOs how search works. Explain it to me. How are pages indexed? How are they analyzed? How is a search query handled? How are queries matched to web pages? Explain ranking. Explain how the knowledge graph works. And so on.
Push back on the SEOs and the writers. Can the writers explain why their expertise is so important? You already know it is, but make them explain it to you anyway.
I bet both writers and SEOs will begin to stutter.
Lastly, if your SEOs are only reading online advice, you are in real trouble! Ask them what sources they have for their information. No research papers? No patents? No books written by Googlers? Stop there. Your SEOs do not know enough.
Search is about understanding what content is about, what the search query is looking for, and how to match the two. It is about whole language. Full stop! Not keywords. It is about topical analysis, linguistics, etc. It is about using AI and machine learning to evaluate search queries and their result sets and how they perform with search users. It is about many many many factors, signals, and metrics. It is a science. Does either side understand this?
So both the writers and SEOs are important. But neither should drive the bus. They both should based upon the application of science, experience, and a little dumb luck every now and again.