My sump pumps discharge onto the lawn outside the house. I notice the
grass there is very lush and green.

I have a feeling that the lush growth is due not just to the grass
getting more water.

I think maybe it is due to the mineral content of the sump water.

I think maybe because the sump water has percolated through the soil, it
has picked up minerals that are good for the grass, whereas rain water
has no dissolved minerals.