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.
I think we need to take all the hypothetical numbers and divide by 4, because the results are not meeting even the conservative projections
Last year, German onshore wind generated 111.9 TWh, or roughly 320 MWh/km2. I can't find the numbers of onshore wind in the US, but the total energy generated by wind power was 453.5 TWh last year. Divide that by land area, and you get ~50 MWh/km2. Because the US has developed only a small fraction of its wind power potential, new turbines can still be built in favorable locations, and capacity factors are high. Germany has to build new turbines in marginal locations, with lower capacity factors.