Hidding a treasure and having someone who doesn't understand geography create the map doesn't seem logical to me, but I've seen worst in the industry.
Do you mean an HTTP API or a programming-language/library API? If the latter, definitely get a developer. If the former, they should at least be familiar with a formal language like SQL. They need to be thinking about the API in terms of the contract (preconditions etc.) it defines between the client code and the API implementation. That's rare if you never used a formal language.
I've been interviewing for a technical writer and some very experienced people, who seem to be greater at the job of Technical Writer, don't have basic coding proficiency.
If my org needs a writer who can code too, should we be trying to find a _developer_ who wants to write too? Coding and writing seem like different careers paths so are we looking for a unicorn?