If they want to use Scrum, then there are three very well-defined roles. Combining SM and PO will not be effective because PO role needs to focus on product decisions. SM activities would distract the PO from that. Likewise for the SM role. Both the roles have different sets of activities and combining both will create conflict of interest.
If the SM and PO were the same person, it’s likely the PO role would dominate. The PO represents the interests of the customer whereby the SM represents the team. The separation offers a pragmatic argument between client expectation vs technical feasibility.