The implementation subject matter expert is not a stakeholder who participates in the prioritization of requirements, because they are not directly affected by the business need or the solution. The project team, the domain subject matter expert, and the project manager are all stakeholders who have a vested interest in the outcome of the project and the value of the solution, and therefore they should be involved in the prioritization of requirements. References:
Business Analysis Expert Certification, CCBA® | IIBA®, section "Who can Become a CCBA?" A Guide to the Business Analysis Body of Knowledge® (BABOK® Guide), section 4.2.5 "Prioritize Requirements"