page 394 PMBOK there is an if statement.
If a project is result of proc activity or is based on an established contract the parties in that contract are key stakeholders.

Good question, but I think it's because high level requirements are defined through the Business Case into the Project Charter, so you may have a general idea that you need a Procurement Contract. But can the Procurement be conducted to where you know who the Seller is, so now you can add them as a Stakeholder?

Let's hope one of the experts can chime in.

What procurement documents do you need when identifying stakeholders? Don't you need the stakeholders to define requirements which in turn provides the foundation for the procurement processes? Which comes first the chicken or the egg?