Ask HN: Our PO defines what a 3rd party API should do in Product Requirements

We are experiencing what seems to be a communication breakdown between the Engineering org and the Product org. The Product Owner believes the implementation details are not his responsibility and that we should not couple our product's abilities with our vendors' APIs. Then he defines Product Requirements that

a) do not fully utilize the potential of the vendors' APIs and

b) ignore the APIs' workflow and limitations.

How do I explain to the PO that even though he is not responsible for the technical implementation, the Product Requirements must utilize and conform to the 3rd party APIs?