Agreed! Thanks, that is certainly the caveat I would add. In real world, monopsonies are however rare to nonexistent
This hypothetical is more an analysis on the constraints (monopoly employer, closed system, static labor supply / demand, static prices) than on the effect of the minimum wage on consumption. It ignores that the minimum wage harms both employers and employees that would be hired absent a minimum wage.
A minimum wage will be inefficient in all situations relative to a taxing more inelastic economic activity and redistributing the proceeds.