It appears that the project has only code for single player.
The Merger Tool does not work properly on the majority of buildings. The buildings which can be properly merged however, lack details on the ground/street floor - which may be overcome by manually setting up the street parts of buildings.
There are dozens of additional maps hidden inside the Buildings folder, containing Houdini buildings, and assembled meshes - another starting point for assembling buildings manually.
Another major difficulty - with UE4 it was possible to drag and drop content between different projects, with UE5 this no longer seems to work (perhaps due to the C++ code involved in the Matrix project)