At what Tech Level? Wouldn't a higher Tech Level have more efficient means of collection power from the sun?
The upper limit is the amount of energy reaching the ground from the star in question. Technology can never improve that number, technology can only improve how much of that number is collected and stored, and technology can never make the collection and storage process perfect.
What McPerth is ignoring, perhaps deliberately, is the fact that his assumptions require the perfect collection of all solar energy across all wavelengths ranging from IR to gamma, the perfect transmission of all that energy to the laser's power pack, and the perfect storage of the same within the power pack in a form which the laser can then use.
He's also ignoring the fact that the max solar number he's using only occurs at Earth's equator with Sol directly overhead and with perfect (again) weather.
For anyone interesting is actually examining the idea instead of simply making shit up, let me suggest this link as a place to start.