Efficiency has almost doubled since 2008 but more importantly efficiency at non optimal angles has improved even more. Put together he's off by about an order of magnitude in the amount of land required.

The other factor that is missed is that solar panels are proving to be surprisingly non exclusive in their land usage.

People doubting there is enough land to be used for solar panels should check how much land is used for parking lots.

Efficiency may have doubled, but the book used a very optimistic 30% efficiency, which afaik is still quite a bit higher than even current rates.

The book used a very pessimistic 10% efficiency for panels installed in large scale solar farms:

https://www.withouthotair.com/c6/page_41.shtml

If a breakthrough of solar technology occurs and the cost of photovoltaics came down enough that we could deploy panels all over the countryside, what is the maximum conceivable production? Well, if we covered 5% of the UK with 10%-efficient panels, we’d have

10% × 100 W/m2 × 200 m2 per person ≈ 50 kWh/day/person.

I assumed only 10%-efficient panels, by the way, because I imagine that solar panels would be mass-produced on such a scale only if they were very cheap, and it’s the lower-efficiency panels that will get cheap first.

Utility scale panels today are commonly a bit over 20%.