On the surface, this statement appears as if I’ve lost my mind. For one thing, a traditional view to Product Owners is as a Product Manager or customer/business stakeholder-facing role. And the traditional Project Manager is more so a planning and execution focused role. The two are quite far apart and seem to have little synergy.
The other factor is traditional versus agile contexts.
There are no “traditional” Product Owners. Usually a Product Manager is in essentially the role but it’s very outwardly and upwardly facing. Once the requirements are “signed off”, they’re not that interested in collaborating with the team until the end of the project.