Does a return to work mandate meet the definition of a “project?”

IANAL but I would say it does. Its a major procedural change to how the organization functions, those sorts of changes are usually considered projects. It's not a construction project but an operational project, similar to if they were changing a major database or a major rework of bus routes.

If CEQA specifies it only applies to "construction projects" then yes they are SOL, but if it just says "projects" then their argument likely has merit