"If you're trying to win over a technical audience, you need to communicate with them on a technical level. You need to understand the technical issues they face and talk with credibility about those issues.
I'm not saying content marketing should consist of verbatim topics from a user manual. But the concepts that you cover in tech docs could certainly be repurposed into larger, more conceptual articles that address strategies around those topics.
For example, a topic in a user guide on "Generating an HMAC Signature for Requests" might be repurposed into an article on using "HMAC Authentication for APIs," in contrast to other authentication protocols (such as OAuth).
As a technical writer, you have the insight to provide substantial, information rich content that will build relationships with technical audiences.
Marketing writing without substance is going to come across as empty marketing — the kind of brochure or product literature that has such a strong agenda it's almost unreadable."
Approved marketing content essentially consists of announcements and tech tips. As such, it's no better than documentation, so why not focus on documentation itself and let marketers (or whomever -- support, BD, training) share direct links to the docs?