I'm... a little dubious about some of their estimates. In particular Ireland, where their light-based estimate is shown as being spot on. Ireland's very high apparent GDP growth is largely structural weirdness (the economy has grown relatively fast once you filter it out, but not at the ridiculous pace that raw GDP figures would suggest), and we definitely haven't been adding lights at an extra-fast pace to keep up with it :)

(More seriously, this seems like an interesting approach for developing nations, but is less likely to give useful results for developed ones.)