1. The target of 16-pin SOIC is Power Adapter based on Opto Coupler Feedback. It means the existing firmware example base on 16-pin SOIC is for Opto Coupler Feedback application. Both AC/DC and DC/DC is supporting Opto Coupler Feedback can use it. Typically, AC/DC have more part number based on Opto Coupler Feedback.
2. Youcan based on Direct Feedback example firmware and change to CYPD3174-16SXQ. After this change, the pre-trim parameters is faked. And the relates EA parameters which is for direct feedback will be enabled.
Based on above clarification, the answer for your question,
Is it correct to recognize that the IC was developed based on AC-DC control as primary?
Or it is possible to control DC-DC?