but you cannot "guess" the complete sequence for any particular individual.

Of course you can. If you have a giant complete pedigree for most or all of the population and you have SNPs or whole-genomes for a small fraction of the members, and especially if it's a highly homogenous population, then you can impute full genomes with varying but still-far-better-than-whole-population-base-rate accuracy for any particular entry (person) in the family tree. They're all highly correlated. This is no odder than noting that you can infer a lot about a parent's genome from one or two childrens' genomes despite never seeing the parent's genome. Your first cousin's genome says a lot about your genome, and even more if one can put it into a family tree and also has one of your grandparent's genomes. And if you have all the family trees and samples from most of them...

(This will not work too well for Kuwait since while the citizens may be highly inbred, they do not have the same genealogical records, and citizens are, IIRC, outnumbered by resident foreigners who are drawn from all over the world and especially poor countries. But it does work for Iceland.)