It depends on your model’s formulation. If we follow the assignment’s, then we can’t remove it. If you would like to discuss the feasibility of removing it, then it’s better to base the discussion on the content of one of those blog posts.
A possible alternative approach to not ever consider cases where r(i,j)=0 is, for example, we have an embedding layer for users and an embedding layer for items, then the model is so built that we only pick positive pairs (i.e. r(i,j)=1) to do the dot product (plus bias) and we minimize the difference between the result of the dot product and the true rating.
Cheers,
Raymond