I think there's a large undocumented migrant population too, who make up a lot of the working classes and this could skew that stats as well.
EDIT it's a lovely place too, Hong Kong (picturesque, pleasant climate, easy to get around). I'd say that would have a lot to do with it as well, once you can pay the rent ...
1. A recent study that found very high life expectancy "blue zones" were highly correlated with poor record keeping and pensions - the strong implication was that it was mostly fraud. Don't think Hong Kong was one of the areas studied, but I think skepticism is warranted. 2. A common thread I've seen in many longevity studies is that social connectedness is one of the strongest predictors of a long lifespan, and contrarily, "lonely people die young." I'd expect that a very close-knit urban area with relatively low migration (e.g. as opposed to NY or SF where there is so much moving in and moving out) would exhibit this social connectedness.