You said the ‘price of Labour decreases with each additional worker demanded’ did you mean the proportion of their costs instead? Or are you referring to an imperfectly competitive market? - because if you are talking about a perfectly competitive market, the price of Labour, including additional units of Labour is the same hence why W also = MR.
I think it might just be an accident with the wrong word/not explained because you clearly know the W=MR because you put that in your diagram
Hope this helped