Correct me if I'm wrong, but the only reason the matrix being sparse is mentioned is because the author stuck a bunch of 0s in the matrices... this solution should work regardless of the sparsity.
Not sure if I completely understand what you mean, but the benefit of a sparse matrix is that zero-valued elements are made implicit by the absence of that (row, column) relation in the database. The drawback is that you need to store a (row, column) for every non-zero-value.
It would be wasteful to use this approach for non-sparse matrices, but it would still "work".
It would be wasteful to use this approach for non-sparse matrices, but it would still "work".