That is partly true. High population density means a lot of roof area. Solar is perfect to put on roofs, you need no extra land. It is basically free (save the investment of the panels which pay off quickly nowadays)
Rooftop solar is easy in suburban sprawl. You don't need extra land, if you have already used extra land for densely built but sparsely populated residential areas. But if you instead have cities, towns, and villages separated by agricultural and natural areas, solar competes against other uses for the land.
> High population density means a lot of roof area.
Up until you start building cities and building with more than one floor. Then you have more people and power usage per roof area. Then you have to put solar panels on some unused area around city to offset that high population density.
