With a sparse system, you might run into dependency chains that effectively make it dense.

Like in an adjacency matrix for a graph, you may have a lot of sparsity, but if you're forced to consider neighbors of neighbors, the adjacency matrix you need is A^2. This can be dense even if A is quite sparse.

It's being downvoted because it comes across as someone arrogantly dismissing an important work because they have no idea what they are talking about. If they instead asked "why is this noteworthy? in my ignorance this seems obvious", they might have gotten a better reception.