I wonder if you sum every countries claimed size, how far off it is from the actual total land on Earth.

Well, this is a pretty perfect usecase for AI (although it's also easy to just get shapefiles from NE and do it yourself, obviously).

Interestingly all of UAE, Yemen and Saudi Arabia have significant discrepancies between the World Factbook and their shapefiles (in every case overestimating), probably all related to not having had a demarcated border in the Rub Al Khali.