I have been a participant in a few debates regarding this question. It comes up because there are 'expectations' of content marketing that may not necessarily be reasonable because the ask is simply not content marketing.
There are two areas where I see this confusion happen the most; Lead Generation and Product Marketing. There is this sort of underlying assumption that because it is writing email copy or writing a datasheet, then the project should be owned by the writers i.e. Content Marketing.
Content Marketing is not Lead Generation
The lead generation, marketing program management team own the inbound and outbound strategy to acquire leads and accelerate them through the funnel. This may include writing email copy, writing display copy, designing Facebook text ads, etc.
Your content marketing team may offer to help with grammar and add any suggestions but the copy development is not their responsibility.
Content Marketing is not Product Marketing
Product marketing owns the strategy to communicate what the product/service is, how it helps and why people need it. production of content that is product related. This often includes writing datasheets, release notes, product web page copy, etc.
Again, your content marketing team may offer to help with grammar and add any suggestions but the copy development is not their responsibility.
What is Content Marketing
What many miss is that content marketing is really focused on content that supports SEO. This group is not writing email copy, writing product blurbs and datasheets, writing text for your display ad or writing tweets. They are executing a strategy to leverage short and long-form content to support the goals of the company.
There are many creative ways a content team can leverage their talents communicating a message from video creation, case study development, long-form papers and blogs.
TIP: Leverage content marketing experts for what they are really good at and allow them to focus there without polluting their to-do list with email copy requests and landing page copy. Involve them early in your campaign development stage and discuss their role.