Sunlight is not free. In order to collect it you must use some land, and that incurs the opportunity cost of not doing something else with that land, such as growing plants for food.
That's not what he means. A solar energy harvester with 0.1% efficiency might still be worth it if the costs are low, it doesn't matter that you "wasted" the other 99.9% since it's unlimited.
It does matter when it comes to CO2 fixation though... if you only get 0.1% efficiency in fixing CO2 with biofuel production, but would get 1% with normal forestation, then there is a quantifiable difference.
I assure you that it would be madness to try to grow food in the places where solar cells are most effective.
Try looking for the "Desertec" project.
The main cost of solar cells are the resources you need to built them and the chemical waste that gets produced
In the process.
